While there's a lot of complaints that could be leveled against the game (ranging from the experience with instances and raids to the likability of newer content) to be honest, none of it is really behind the decline.
Simply put, all MMOs have a finite lifespan. And in this case, WoW is past its peak, and, for better or worse, headed towards its death. There isn't anything Blizzard COULD do to stop this from happening; at best, they can slow it down with expansions, but as we've seen, by now it's not very effective; while Burning Crusade helped accelerate its growth through the (then uncharted) multi-million-subscribers territory, and Wrath of the Lich King allowed it to hit a record-setting peak of 12 million... The game couldn't grow beyond that, and started to decline.
Ignoring the temporary shuttering of WoW China (taking 5 million subs away for a brief period) the game sank back down to nearly 10 million before Cataclysm pushed it back up to the 12.0 mark seen in the WotLK heydey two years prior... But after that a more steady decline occurred, nearly doubling: the same 10 million mark was dropped only about a year and 3 months after Cata's release, ending a bit below that before MoP launched...
But by then, the expansion couldn't restore the game; it was already becoming old-hat, so it only pushed back past 10 million... And THAT rebound evaporated almost instantly; a lot of them simply quit once they finished the new content, and already, 3/4 of a year since then, we've dropped to 7,700,000 subscribers, or around a million a quarter.
At this rate, (again, for better or worse) the game may likely shut its doors within 3-4 years, or at least undergo a transformation to an entirely different form (such as entirely freemium) after dropping to around 1 million or less subscribers by then. But by then, there will be so much fewer current players that people won't care as much... And a >10-year lifespan will be impressive for an MMO, setting a new record for an "ended" game.