That's a dumb criticism, since the merchandising of a brick-and-mortar bookstore typically does the same thing as that of an online retailer: presents whatever the store is trying to get rid of to everyone up-front, and otherwise presents people with things related to what they are searching for specifically, avoiding exposing them to the rest.
You can actively choose to browse (wander through a physical bookstore, click different categories for an online one) for the purpose of providing yourself opportunities for serendipity, but no bookstore is designed to encourage serendipity, because anything that does that is a terrible UX for most customers and leads to confusion, frustration, and not buying, and the store going out of business.
Book discoveries, like album discoveries, happen better in stores. It's not nostalgia, but just the way the physical world works. (Think of how music lovers connected with album covers in the vinyl world.)
Just because something is "the old way" and there is nostalgia for it doesn't automatically make it worthless.
For me, Spotify is the best way for me to discover new music. Usually through the thematic aligned playlists or unofficial playlists curated by other users.
In part, I think, this is because Rdio implemented social features poorly. I find music based on the music, not on what's popular. And it's willing to recommend things off the long tail that I'd never find in a store—one of my favorites was an album from a French band that never released anything else and probably didn't make it to a single US store.
The same isn't quite as true for bookstores, perhaps because popularity plays a somewhat smaller role than for music. But I could still see online recommendations being better for depth, especially if I wanted to dive deeply into some genre and explore obscure authors that never really made it.
If there's any truth to the complaints, I think it's that they speak to a demand for more variety in "third places" - somewhere to meet friends and serendipitously run into new people outside of the house. Restaurants and bars are fine, but I think people miss other kinds of commerce - retail used to add that variety. Perhaps the eager adoption of work share spaces speaks to this. I think there are more business opportunities catering to this innate human need.