Q u o t e: We still think that's true for a MMO in which thousands of players co-mingle in a persistent world and vie for supremacy in eSport competitions or 'world first' boss kills in raids. Neither of these are true though for a co-op action RPG.
I'd love to see you define "persistent world" in a way that soundly differentiates World of Warcraft's 3D environments from Diablo's 3D environments. Oh, that's right, you can't because you're speaking a in fallacious manner. I can, however, argue that Diablo 3 shares a staggering number of traits with a variety of free to play 2D MMORPG's.