Why I Hate Forced Categories
ON DESPISING GENRES is a decent reading of why I hate forced categorization, where your "Hard Rock" may be my "Classical":
Tolkien is famous, so Tolkien gets shelved with Realism. But almost no sf gets deghettoised this way, because few librarians read enough sf or fantasy or know enough about it to pick out the books of "genuine literary value" from the commercial schlock.
Commercial schlock is not limited to genre fiction, and so fiction of absolutely no literary merit at all, commercial junk realism, gets shelved with Austen and Bronte and Woolf, while sf and fantasy of real merit and real interest gets treated as junk by definition. No wonder writers like Kurt Vonnegut deny strenuously that their sf is sf