I don't know if GW2 will out-subscribe/userbase WoW, but it will most likely be better. Hell, I'm not sure if WoW is even better than recent offerings like Aion or Warhammer, mostly because I haven't played the new offerings. WoW is so flawed it would be easy to out-class, the issue is that PC userbases don't move on easily and userbase is major part of the draw for online games.
Right now Blizzard seems cocky enough to make regurgitation their status quo. Re-use Naxx, reuse item models, just re-use the entire original game for Cataclysm. Maybe that will make people finally turn on them. The MMO userbase is so inherently lazy and depressed that its hard to 'wrong' them enough to switch sides. It's not a matter of making a better game than WoW, because thats already been done. It's a matter of Blizzard finally disrespecting the users enough to get burned by it.