Library Technology major here. Usually when you look in a book store you find a natural ebb and flow to which books are listed which way depending on popularity. Horror for instance has become more of a sub genre in recent years rather than a category of its own since most books identify as thriller or mystery which are more popular in general at the moment. Technically the books could be put in Horror and this wouldn't be a miss identification but it wouldn't serve to make it visible to the largest potential receptive audience. Conversely Supernatural titles have gained enough to break out of being identified chiefly as fantasy or romance titles.
Genre is a living thing that should be responsive to market forces. Its not going to happen but those thirty tags as Evan mentioned should exist and be dynamically sensitive to search quarries. If you have a romance steampunk novel and steampunk is trending then your book should already be identified as steampunk because that's more effective, if only temporarily, than romance. If you have a top 3-5 feed of the most popular genre's that actively updates on the front page then users get to decide what is most popular and see the readily available content reflect that. Do I think romance and comedy will dominate this? Yeah but the other spots will likely change hands fairly often with a couple regulars and then some niche stuff that's blowing up for a while.
Again, not stuff I think will happen. This is just what goes on in my brain when I'm staring at the front page of my library systems webpage and realize I am but an ant in fortunes place and can do nothing meaningful to change it.