Technology changes, consumer attitudes change, etc. I mean, 10 years ago, the "profile" of the typical MMO player was 15-25 year old males that fell into the "geek, dweeb, nerd or loser" category. "Cool" people didn't play them, they played console or FPS games.

WoW really did change that dynamic, making MMOs much more socially acceptable in the American market. And with that came a whole new set of expectations from the consumer base. People expect a game to be something they can invest more than a few months in. They expect it to evolve. And they also expect it to be something they can go back to later. As long as WoW can maintain that, it will keep a good population.

I don't imagine that it will remain the reigning champ of MMOs forever, but it's still got some steady lifespan ahead of it. The number of MMOs out there just keep growing, so there's some inevitable splintering of the playerbase, but people are also hard pressed to let go of something they've invested a good amount of time in. Sure, a good chunk of people left after Cata launched, but I'm sure a fair number of them came back with 4.1 or will come back with 4.2.

I'm not really seeing a single game just coming in and decimating WoW. It'll take a combination of things to knock WoW off its throne, not the least of which is time, IMO.