Hmm...I'm playing GW2 atm, am somewhat satisfied with it, but will be pushing the game off to the side nonetheless for MoP. Guild Wars 2 is still brand new, you can't possibly think that everyone that bought it is going to play that and nothing else, but given time people will change their minds about it. Plus, a lot of people bought it simply because it's B2P and had a lot of hype behind it rather than to all-out replace their current MMO.
There is no doubt in my mind that WoW subs will shoot over 10 million again in early MoP, maybe not for long but for a quarter anyways, absolutely. Too many people still have interest in the game plus lots of time invested to not try it....and, face it, what has WoW offered in the past 8 months? Absolutely nothing new, nothing to do but crappy DS and with plenty of new games out of course people are going to stop playing, some permanently and some only temporarily. Plenty of people still like PvE encounters, for all the shit Blizzard gets with theirs, really their only rival as far as an quality endgame PvE goes is Rift, so people will certainly miss that and want to come back for that purpose.
I personally don't give a shit which MMO gains/loses the most. The more competition, the better each will be as they try to outdo each other which can only make for better games, regardless of which one(s) you like. And as long as you enjoy the game you are playing and there are enough subs to sustain it, there really is no reason for you to care either.