I don't think WoW will be "dead" in 20 years. Given the huge player base now, I'm sure there will still be quite a few players remaining in a decade or two, so Blizzard will continue to maintain the game, reset arena seasons every now and then, etc., even if they stop releasing new content.
I'm sure Blizzard's goal is to shift many players into its next MMO, but that doesn't mean it will just abandon WoW.
I've come back to WoW just yesterday after finally deciding to get Mists. Actually this is my third time coming back, once after each expansion has had some time to simmer down.
From what I can tell, the game is still very much alive and I don't see it dying out any time even remotely soon.
As a side note, I'm glad I chose to wait a while after MoP launched. I don't think the Pandaren starting area would have had the same impact on me if there were a swarm of fresh characters running around the place. Very well done starter zone IMO. Better than Worgen and Goblin if you ask me.
Anyhow. I hope you enjoy the game as much as I have. It really is fantastic, regardless of what any haters say anout it.