You're right, it's definitely a factor. I don't care how good any game is or what they add to it, it eventually loses the new game smell. I'd ballpark 5-6 years is about the mark I'd peg most MMOs to hit their peak and start declining as other games come out with a fresh world to excite players.
It doesn't mean Blizzard has to accept the inevitable, of course they can still strengthen the game, but I'd be willing to wager you won't find an MMO that maintains players in its second 5 years as it did in the first 5 years, regardless of which MMO and its player base size.
Honestly, TBC and Wrath I wouldn't say the game was old. They were the first two expansions. We're now at four expansions and heading towards the second half of this one with a fifth's announcement on the horizon. Yes, the game is getting old.
There's also more competition than ever now and the F2P model is definitely growing, allowing players freedom to hop from game to game without having to stick to any of them.
But even moreso, it's an aging game in a rapidly changing market and this specific genre is definitely in flux right now.
Occulus Rift compatible WoW incoming?