I don't think Rift will fail by any measure. It has too large of a playerbase looking at it right now. As long as it is actually a finished product when released, it will be very successful. 90% of MMOs fail because they get rushed out the door.
On a different note, everything I hear lately indicates SW:TOR is headed for a huge failure. I lost faith in that game as soon as I found out mythic was making it(Yes, Mythic. Not Bioware. They are just using the name.) A shame too, because it had such potential. We'll see though, in my opinion it has a chance if it gets delayed into 2012.
Currently playing: EQ2(AB)