I think your claim is a bit absurd. One of the first requirements for literature to be deemed great is that it stand the test of time. I believe Kerouac and the other Beats are great writers, but their work can't truly be called great for another 30 years or so. If it's still that popular then, then your argument is valid. Someone could just claim the Beats are so popular because they are fairly recent. That they're only popular because they wrote in a time many of us were alive during. On the other hand, writers such as Twain, Emerson, Thoreau, Hawthorne, etc. can be called truly great because their work has been held in high regard for over a hundred years, and by many generations. Kerouac has been accepted by 2 or 3 generations at most.