If the first half of the volume was an alternately dreamy and dreary description of the romance of the Guermantes, the second half is an attempt to contextualize it and explain Marcel’s severe disillusionment with them and their place in society. Their charm, he says, lies in the following:

The Guermantes were not only endowed with an exquisite quality of flesh, of hair, of transparency of gaze, but had a way of holding themselves, of walking, of bowing, of looking at one before they shook one’s hand, of shaking hands, which made them as different in all these respects from an ordinary member of fashionable society as he in turn was from a peasant in a smock. And despite their affability one asked oneself: “Have they not indeed the right, though they waive it, when they see us walk, bow, leave a room, do any of those things which when performed by them become as graceful as the flight of a swallow or the droop of a rose on its stem, to think: ‘These people are of a different breed from us, and we are the lords of creation’?” (455)

Remember that this comes in the context of Marcel having rejected the so-called intellectual social life, thinking it to be false, misleading, and a waste of time. Earlier, Proust had extensively celebrated the pleasure of frivolous association with silly young girls over any would-be meaningful conversation. He chooses to restate this theme at length early in the second half of The Guermantes Way, at a moment of frustration and boredom with Saint-Loup:

I had reached the point, at Balbec, of regarding the pleasure of playing with a troop of girls as less destructive of the spiritual life, to which at least it remains alien, than friendship, the whole effort of which is directed towards making us sacrifice the only part of ourselves that is real and incommunicable (otherwise than by means of art) to a superficial self which, unlike the other, finds no joy in its own being, but rather a vague, sentimental glow at feeling itself supported by external props, hospitalized in an extraneous individuality, where, happy in the protection that is afforded it there, it expresses its well-being in warm approval and marvels at qualities which it would denounce as failings and seek to correct in itself. (409)

Again, the emphasis is on how friendship saps the soul, how it takes up valuable time and spiritual resources. Again, Proust stresses that people are not so malleable as to gain insight and growth from a supposed interchange of ideas; rather, he insists, it leads one to water down one’s self-possessed ideas in the service of illusory connections.
Coming as it does after long passages about how people’s passions and desires can turn on a dime (Marcel himself, Swann, Saint-Loup, Albertine, etc.), the repeated emphasis on a fortress mentality towards one’s own personality seems incongruous. If past feelings can become as alien to one’s own self as a country to which you’ve never been, suitable for study but never knowable immanently, why protect the inner self from such violence that is visited upon it by friendship?
The book is not a treatise and Proust is attempting to explain, iteratively, what he sees around him, which encompasses both radical changes in one’s own self at the same time that clear spirits prevail throughout, as with Elstir and Bergotte. (Though Bergotte is showing his age and is a shadow of his former self.) Artistic creation takes place over time and may evolve, but Proust evidently advocates a hermetic mentality towards such an act preserves whatever spirit informs it, rather than allowing it to be corrupted by the standards of the day.
It is the standards that Proust seems most irritated with by the end of The Guermantes Way. His attacks on the Guermantes revolve around the sheer banality of their culture, and the relentless conformity that informs it. There is both disappointment and anger in his reaction, and the two responses aren’t quite synchronized. The disappointment is one of the larger themes, one of having chased after something that seemed distinct and unique, but turned out to be the same old crap:

After having scaled the inaccessible heights of the name Guermantes, on descending the inner slope of the life of the Duchess, I felt on finding there the names, familiar elsewhere, of Victor Hugo, Franz Hals and, I regret to say, Vibert, the same astonishment that an explorer, after having taken into account, in order to visualize the singularity of the native customs in some wild valley of Central America or Northern Africa, its geographical remoteness, the strangeness of its place-names, of its flora, feels on discovering, once he has made his way through a screen of giant aloes or machineels, inhabitants who are engaged in reading Voltaire’s Merope or Alzire. (545)

However tedious I found the Guermantes’, however shapeless I found the endless descriptions of their mores and unique manners, I can sympathize with this crushing despair, as well as the miserable sense of embarrassment that accompanies it, the sense of having bought into a system that has ultimately provided no benefit whatsoever, not even novelty. That it happens without the standard plot twist of a societal mishap or misunderstanding, or some patently unjust act, leaves the focus firmly on the development of Marcel’s own views of the culture in which he has invested.
Yet there is petty anger as well, and Proust’s deployment of it is too vicious to make the haughty, resigned disappointment of the above passage ring entirely true. A few samples:

As though corrupted by the nullity of life in society, the intelligence and sensibility of Mme de Guermantes were too vacillating for disgust not to follow pretty swiftly in the wake of infatuation (leaving her still ready to be attracted by the kind of cleverness which she had alternately sought and abandoned) and for the charm which she had found in some warm-hearted man not to change, if he came too often to see her, sought too freely from her a guidance which she was incapable of giving him, into an irritation which she believed to be produced by her admirer but which was in fact due to the utter impossibility of finding pleasure when one spends all one’s time seeking it. The Duchess’s vagaries of judgment spared no one, except her husband. He alone had never loved her…M. de Guermantes for his part, pursuing a single type of feminine beauty but seeking it in mistresses whom he constantly replaced, had, once he had left them, and to share with him in mocking them, one lasting and identical partner. (489)
The life of the Duchess was by no means easy. M. de Guermantes only became generous and human again for a new mistress, who would, as it generally happened, take the Duchess’s side. (500)

Nasty stuff, and Proust continues with it at length, expressing the same vigor in his disapproval and gossip as he did in his infatuation in the first half of the book. I remarked earlier that Proust reminded me of The Good Soldier in the way that the narrative shifted unreliably to reflect the currents of the narrator’s thoughts. The Guermantes Way tells a very simple story of the extremes of enchantment and disillusionment, but it is the credence that is built up in both parts that prevents the final judgment from being wholly negative. In light of previous developments of the book, his disgust is too calculatedly over the top to displace completely the earlier aesthetic joy (even if I found it fairly dull). In fact, Marcel’s arc closely mirrors that of Swann with Odette, Saint-Loup with Rachel, and to some extent, Marcel himself with Albertine:

Mme de Guermantes’s mind attracted me just because of what it excluded (which was precisely the substance of my own thoughts) and everything which, by virtue of that exclusion, it had been able to preserve, that seductive vigour of supple bodies which no exhausting reflexion, no moral anxiety or nervous disorder has deformed. Her mind, of a formation so anterior to my own, was for me the equivalent of what had been offered me by the gait and the bearing of the girls of the little band along the sea-shore. Mme de Guermantes offered me, domesticated and subdued by civility, by respect for intellectual values, all the energy and charm of a cruel little girl of one of the noble families round Combray who from her childhood had been brought up in the saddle, had tortured cats, gouged out the eyes of rabbits, and, instead of having remained a pillar of virtue, might equally well have been…the most brilliant mistress of the Prince de Sagan, But she was incapable of understanding what I had looked for in her–the charm of her historic name–and the tiny quantity of it that I had found in her, a rustic survival from Guermantes…A misunderstanding that is entirely natural, and one that will always exist between a young dreamer and a society woman, but nevertheless profoundly disturbs him, so long as he has not yet discovered the nature of his imaginative faculties and has not yet resigned himself to the inevitable disappointments he is destined to find in people, as in the theatre, in travel and indeed in love. (522)

In a self-pitying manner, he seems to shoulder some of the blame on himself, just as Swann was painted as having built Odette up into an internal image wholly unlikely the actual woman. The slightly obnoxious tone beneath it (the combined forces of disappointment and anger serving to undermine each other) indicates, however, that Marcel has not really been damaged, not to the extent that Swann was. Disappointed and annoyed, but not mortally wounded. Compared to the drama of Swann’s infatuation and despair, it is calculatedly smaller. And it is with this echo firmly in mind that Proust brings Swann back at the very end of the volume, to drag the book once more back to serious topics. Swann by this point is easily one of the most likable characters, and it’s pretty obvious why he hasn’t had much to say for the entire volume. And his announcement in the final pages puts an uneasy stamp on what the decorative flourishes that have made up the last 600 pages. He declines an invitation to go to Italy with the Guermantes, and having taken offense, the Duchess demands to know why:

“Very well, give me in one word the reason why you can’t come to Italy.”
“But, my dear lady, it’s because I shall then have been dead for several months. According to the doctors I’ve consulted, by the end of the year the thing I’ve got won’t in any case leave me more than three or four months to live, and even that is a generous estimate,” replied Swann with a smile.
“What’s that you say?” cried the Duchess, stopping for a moment on her way to the carriage and raising her beautiful, melancholy blue eyes, now clouded by uncertainty. Placed for the first time in her life between two duties as incompatible as getting into her carriage to go out to dinner and showing compassion for a man who was about to die, she could find nothing in the code of conventions that indicated the right line to follow; not knowing which to choose, she felt obliged to pretend not to believe that the latter alternative need be seriously considered, in order to comply with the first, which at the moment demanded less effort, and thought that the best way of settling the conflict would be to deny that any existed. “You’re joking,” she said to Swann.
“It would be a joke in charming taste,” he replied ironically. “I don’t know why I’m telling you this. I’ve never said a word to you about my illness before. But since you asked me, and since now I may die at any moment…But whatever I do I mustn’t make you late; you’re dining out, remember.” (618)

The back and forth continues for a while until they do go off to dinner, with the Duke complaining about how difficult the Duchess is:

The Duke felt no compunction in speaking thus of his wife’s ailments and his own to a dying man, for the former interested him more and therefore appeared to him more important. And so it was simply from good breeding and good fellowship that, after politely showing us out, he shouted in a stentorian voice from the porch to Swann, who was already in the courtyard: “You, now, don’t let yourself be alarmed by the nonsense of those damned doctors. They’re fools. You’re as sound as a bell. You’ll bury us all!” (620)

And so the volume ends, with a scenario contrived to make the Guermantes look as bad as possible, not able to express a bit of sincere concern for a terminally ill man. Whatever their redeeming aesthetic value, it’s a fair dead certainty that Proust has his readers on the same page at this point, and ready to move on.