It would probably lost a lot of readers, especially among young people. If you remove pure fun please reading as an option, less of them will find themselves in mood for something more serious.
I think the idea that art/literature is "ridiculously subjective" is mostly on the west side of the Atlantic. In ole Europe we tend to take our literary canon seriously, and don't think that all taste is equally OK. Of course individual readers and critics might prefer X over Y, but the value of great works and authors is not doubted in aggregate.
E.g just because some people like Bieber and others like Mingus, doesn't make Bieber as just good as Mingus. Music/literature/etc is a craft (in which one can tell a good from a bad practicer, even if he is not "into" the final artifact) rather than something like different ice cream flavors.
Not a joke at all. Especially if HPMOR is the best example one can come up with (or one of the best).
I mean, not even the actual Harry Potter is any work of literary genius. I'm talking about Melville, Nobokov, Joyce, Dostoyevsky and such. It's not like literature would lose anything important if HPMOR wasn't available.