Here’s a quote from the game’s lead designer, Tom Chiton, in a recent interview.
“For Blizzard it makes sense [to go free-to-play] at some point. But a lot of the risk is in making that transition. You hear stories about developers going free-to-play and getting double the number of players, but you don’t always know it works out that way and how long it stays that way. We really don’t know what the rate is before people drop off and lose interest.”
I’m not sure if WoW could possibly be as good anymore if it went F2P. I think it would make me and many others lose quite a bit of interest in the game. Luckily this seems like it won’t be happening soon, so there’s plenty of time for Blizzard to change their mind.