Some people do stop subscribing, yes, while many just continue paying but don't log in much or join any duty finder queues for various reasons.

People run out of things to do in the game and even the best items you can get now will be quickly invalidated upon the release of an expansion. Even as soon as the expansion does come out, you may have some difficulty getting into older dungeons and such, as people want to do all the new ones just released. A couple weeks later though, the roulettes will be filled again.