I wouldn't be so sure. First off, they list 10.2M as the current subscriber base, taken times a conservative estimate of $13/mo (the every 3 months plan, IIRC) and that's over one and a half billion dollars a year. Second, they are rumored have a new MMO in the pipeline - suppose everyone that ever tried WoW returns to dip into that bucket. I think that, on the contrary, the era of Blizzard printing money is still going strong.
I don't know if $13 is conservative. $9/mo is the most expensive plan available to me. Presumably in places like China people pay by the hour and therefore way less than that.
suppose everyone that ever tried WoW returns to dip into that bucket
Not a reasonable way to estimate potential demand.
Maybe something closer to "Some percentage of established MMO gamers who now play WoW or Star Wars Online will try it according to how appealing it looks and how well it's marketed and priced."
I think I've met people who've committed to do that, but for every one of them I've probably met many more who just got bored with it or wandered off to another game.
Sure not claiming it's a majority, but I could imagine it being 5-10%, which is still enough people to invalidate the original argument of "they'll get everyone who tried WoW" back.