No one will.
Seeing the success of WoW, there are a bunch of competitors in the market, and if WoW ever slips away, people will be absorbed into the MMORPG that best suits their playstyles. Some people like PvP, some like realism, some like RP, some like individual and involved quest lines, ease of play, etc.... WoW came out and aggressively stole the majority of the customer base, and did pretty much everything better than everyone else, and most importantly, did it with minimal issues. I've played games that are head and shoulders above WoW in many of the respects above, but they were introduced after WoW had established its base, and are plagued by bugginess that is just unsat. People don't want to go learn a new game when their entire "friend" base as well as untold numbers of hours are vested in a game already.
Personally, I left WoW just before ICC came out, had an 80 lock in full T9+, but the game was just getting too boring, cartoony, and every expansion was pretty much just a re-hash of the one before. I know "if it ain't broke, don't fix it", but I'm tired of paying so much money to them to only re-skin and re-order raid boss fights, with a crappy grind to top level again.
I like AoC a lot, especially now that RotGS is out. WoW won't be "toppled" until they stop coming out with stuff for it. Their fan base is too loyal. However, that doesn't mean that all other MMORPGs are shit. I think there's a finite amount of time left for WoW, and if Blizz has a follow-up which:
a) can separate itself just enough to not be a re-skinned WoW
b) comes out on time, and
c) launches without major technical hitches,
it's their game to lose. People know what they're getting when they go with Blizz. I don't expect them to screw it up, and I don't suppose they intend to let the WoW base completely disappear before putting out a satisfactory replacement.