Even if blizzard loses a few customers because of this they'll still be just fine I'm sure, but I really do hope ESO is good enough to take away from the WoW/Starcraft population.
Blizz has already lost 2 million WoW subscribers just from putting out shitty expacs. Their all time high was 12-13 million in LK, its now back down to 10-11 million subscribers. If ESO can deliver what everyone is hoping for, I expect to see WoW drop to at least 8 million users, and I wouldn't be surprised to see ESO make it past 5 mill subs in the first 365 days after actual release. Only if they can deliver though, because theres been a lot of games that were supposed to do the same thing but couldn't deliver...
Still, fingers crossed!