SW TOR is based off the KOTOR games and still very similar
those games were a massive success
and from what ive heard you can play all of TOR like that (ex. solo)
So that alone should make it a success

Will people stay registered? who knows.
but if they do they will definitely be making enough money to afford to keep up the voice acting.
And unlike the other games its an MMO. which -is- a threat to the game.

Rift did drain a bunch of people away. Probably a good half or more of wutai's original endgame (the first ppl to kill most NMs and Dodore) left for Rift. And those were the "elite" players who pushed jobs and crafts to 50 while other ppl were still stuck at 30.

Other MMOs always have the potential to drain away players. Especially dissatisfied hardcore ones who have nothing left to do but grind.