I know it's hard for people to believe, but it's very likely that SW:TOR will actually surpass WoW. Not immediately, mind you, but over the course of 2-3 years.
When players get tired of WoW, they tend to drift to other MMOs, such as Age of Conan, Warhammer, and (most recently) Rift.
However, the player has no reason to keep playing those games once they get tired of them. Those games are trying to compete with a gameplay style and fantasy setting that Blizzard has already perfected. There's a reason WoW is immensely popular. Blizzard has the formula down.
WoW got where it was because it entered a market where mediocrity was, for some reason, acceptable at the time. They succeeded because they produced a decidedly superior product that drew in an audience that no one even knew existed.
But now, BioWare is catching the market at the perfect time, with the perfect product. EA isn't stupid, even though they might seem it at times. The reason that "development" has been taking so long is that EA/BioWare is waiting for players to get tired of Cataclysm. There really has been no better time to enter the market, except for when WoW entered it.
But whereas all the other entries in the genre came off as little more than clones of the successful WoW formula, BioWare actually has a product that will combine a recognizable license with (presumably) superior gameplay and a lasting story.
I've played many of the post-WoW MMO's (Age of Conan, Warhammer, etc.) and what shocked me most about them is how they essentially just copied WoW's formula but did a bad job of implementing it. The reason these games can't sustain a large playerbase is that they just assume that, for some reason, players will prefer their game over WoW despite inferior gameplay, which is all those games have going for them.
Sure, LOTRO and Guild Wars placed a sizeable emphasis on the storyline, but that's essentially as far as the genre has come in terms of storytelling. People underestimate the value of a well-told story on keeping people playing. Text boxes just don't do it in this day and age. By introducing a strong storyline with full voice acting, BioWare will be able to keep players interested in what goes on in their world.
Combine BioWare's famous dialogue/storytelling with the absurd marketing power of the Star Wars license, and you have the only type of game with the ability to usurp WoW's position as the top MMO on the market. I have very little doubt in my mind that we're going to see a huge shift in WoW's playerbase within a year of SW:TOR's release.
Of course, SW:TOR could definitely be knocked out by Titan. No one really knows what Blizzard has up their sleeves, and it is very possible that they're developing Titan as a direct counter to The Old Republic. I wouldn't doubt it at all.