How many of the 4,5 million ex-WoW subscribers do you think are sitting on the fence waiting for another MMO to equal or better WoW in quality? When the EA people reckoned the potential MMO market would be over 20 million for a subscription based MMO, their math wasn't wrong, their game was. If an MMO can deliver high quality content and quick patches, there really isn't anything implying they can't have 5-6 million subs 3 years into the game. Now, the company has to work hard AND be patient to achieve this, which unfortunately is very rare in today's gaming industry. My bet is also that ESO will be a F2P or B2P with some sort of cash shop within 1,5 years of release.
But thinking WoW players won't quit their game for something better/different is naive at best and recent sub numbers confirms that. Blizzard lost touch with their gamers and tried to appeal to everyone, but it turns out "everyone" don't like MoP and they got alot to correct come The Dark Below. I'm for one is expecting Blizzard to pull something amazing out their sleeves to counter the impact EQ Next, Wildstar and TESO will have on WoW. I presume you read the news about Blizz not hiring for Titan anymore? I'm thinking the Titan team is more or less working on WoW now. It might be hoping too much, but I believe we haven't seen the last of WoW either.