Gopnik makes clear that Proust started off as an upper-class dilettante, or a writer who only stayed in the superficial. He calls Proust "a sort of imporobable Bell Époque Tolkien," and makes a clear distinction between other great writers, such as James Joyce and the aesthically-focused work that Proust published. Gopnik writes:

His terrain is, rather, the strangled loves and pains of a small, fashionable circle, with much of the novel spent with the narrator going back and forth to beach resorts and feeling things, and many more pages, particularly in the middle books, where he simply takes trains, feels jealous, then feels less jealous, then more.

Of course, there is nothing at all wrong with this; Gopnik just wants to question the status of Proust in the modern day. He continues:

Proust is least interesting for his philosophical depth. The profound bits in Proust are the most commonplace, while the commonplace bits—the descriptions, the evocation of place, the characterizations, the jokes, the observations, and, most of all, the love stories—are the most profound. His is the most militant tract of aestheticism ever attempted.

Once we have this in our minds, Gopnik goes on to praise Proust's aestheticism. He comments that "his mind moved aexactly as his sentences do...without being halted by other people's literary rules." And "he offered a picture of a particularly beautiful place and period in the world's history." It would therefore make sense why Proust is so popular and has such a cult following. People love Paris and the bourgeois lifestyle. Over time, people conflated their interest in his wildly aesthetic work with a philosophical importance (with people even comparing his theory of time to Einstein's, ridiculously).

Gopnik's take is a good one, and worth picking up for a short read.