The majority doesn't have to go anywhere. Just the most engaged people (the ones who will be visiting /r/place, for example) who provide the value that keeps the other ones visiting and viewing ads. Once they're the only ones left the site will be basically a zombie.

It takes time for giants to fall. Digg is still around to this day, and Reddit will likely be around a decade from now too. It just won't be the "front page of the Internet" any more.