Well, considering the original King, EverQuest, is STILL alive and kicking, 12 years later (though granted not at nearly the population levels as at it's peak), I have no doubt that WoW will survive at least another 10 years from now. When will its population start to drop? I would estimate soon, because quite frankly, it is the same old thing, expansion after expansion, and people will just eventually tire of the formula, and I think we're seeing a very small sign of that with Cataclysm. They just recently announced that they had their first overall drop in subscription numbers (granted, again it was very small). I'd say starting about a year from now, with the release of The Old Republic and some other MMOs, that you'll start seeing a noticable population drop, but WoW will remain the "King" for at least another 2-3 years, minimum.

Thats just my opinion based on playing MMOs for 1/3 of my 35 years on this earth!