WoW will never be what it once was in it's peak, during WotLK... but it's STILL doing strong --- almost 7m, PAYING SUBSCRIBERS I might add... not free loaders off F2P's, for a decade old game is still pretty damn impressive.
Numbers will surge again for WoD - probably more so than MoP due to TBC fans wanting to revisit familiar zones.
But the game is simply too old and people who've never played it at very unlikely to jump in due to graphics and/or being waaaaaaaaaaaaaay behind putting them off with 5 expansions in.