I got a feeling. That tonight's gonna be a good night. That tonight's gonna be a good good night.
I hope I have that stuck in your head now, just like the song is stuck in my head after reading the OP's thread title.
Anyhoo, considering Rift is still going strong and that was basically WoW with extra points placed in Realistic Graphics, I'd say SWTOR will do pretty well. It's got a lot of strong things going for it: a brilliant developer, a strong and rich IP with an incredibly extensive universe, and a well-timed release date: as much as I love WoW, I know full well that it's currently hitting a lull, just as it always does when a new expansion is on the horizon. SWTOR is hitting at just the right point to grab many subscribers from WoW.
Will it be able to keep those subscribers though? We can only wait and see.
I'm sure both MMOs will be able to live alongside one another. They'll just provide strong competition for each other, which can only be a good thing for us subscribers.