The movie, expansion and visual upgrades will help, but the ship has sailed and Blizzard need to start preparing for its decline instead of trying to postpone it by ripping out difficulty that made it so much fun in the first place.
I think a lot of people are missing the point with WoW, including the corporate people who hire the devs. I joined the game during BC, and for the most part, the subscriber chart matches my own opinion of the way the game is going. BC was the best IMO, with WotLK being decent too. In both cases, Blizz made it feel like my character was really there, part of the fight against the "big bad," whether that was the Burning Legion or the Lich King and his undead army.
It all started to go downhill with the new devs. Cataclysm was a disaster, and not just in terms of what was happening to its characters. It seemed like the devs wanted to um... wet everyone's Wheaties... make sure everyone gets a little bit to make sure they know the devs despise them. No sacred cow would remain unharmed; some bit of the game each player enjoyed would be hacked out for no apparent reason. Harmless little things that added to player enjoyment were removed. The "war on fun" has been an ongoing accusation against Blizzard since that time, and not without justification.
The reason for the mass exodus starting with Cata was not that everyone suddenly got tired of this game that had by then been around for six years (and been growing nearly all of that time). It was because it was the first expansion developed by *****s who had no feel for the game, and no respect for it, its players, or its lore.
While it had always been a part of WoW, Cata was when the Blizzard habit of nerfing a class or spec into oblivion, then gradually building it back up, began in earnest. Bit by bit, Blizz took away something each player enjoyed in his own way, and usually for the most inane reasons possible. Eventually, the nerf bat hit each of us... the spec that was your favorite, the reason you keep playing, one day was obliterated, with the play style you enjoyed completely changed for no reason other than to change it. Often times, that change coincided with a bonecrushing hit from the nerf bat at the same time.
There's only so many times they can do that to their player base before people get fed up.
After Cata came MoP... which featured the "big bad" of... anger. Yeah, the living embodiment of negative emotions. We'd all thought that Cata's Deathwing was a lackluster baddie (despite the massive amount of damage Blizzard had done to the game in his name... all your favorite leveling zones are all wrecked now! Uh, but it wasn't new Blizz devs marking their territory... um, it was Deathwing that did it! You should be really mad at Deathwing now!)
WoD started strong. The Iron Horde (the big bad of WoD) is front and center again, which is good (although how it relates to the present my character lives in is still a mystery). It seemed like a winner in the first days. A lot of people were saying it was the best expansion since BC in chat... some even said it was better than BC. The euphoria quickly wore off when people starting hitting the level cap, realizing that the good leveling experience didn't suggest in any way that the end game had much end-game content. The new PVP zone, Ashran, has been a poorly executed, cobbled-together mess since the beginning, and the efforts to fix it have only made it worse. Dungeons, even heroics, don't drop any gear worth the time to get them, and there are no marks of whatever to make it worth people's while to run them regularly. Progression raiding is just about gone... I don't do that myself, so you'd have to ask one of them why that is.
That's why I don't think people are leaving because WoW is tired and old. They're leaving because Blizz has disappointed pretty much every player with callous, poorly-thought out decisions that really make you think Blizz hates its customers. Not just once, but repeatedly. If there is a bone-headed move that is sure to enrage players, you can bet Blizzard thought it was a good idea, and rushed it into the game without a thought (ignoring the words of wisdom invariably offered by the long-time players who still test such things on the public test realm).
WoW has immense gravity because of its size. It pulls people who have quit back in, because they often meet someone else (IRL or otherwise) who plays WoW, and they think it would be fun to play with this new person. If WoW is any kind of a decent game, it can again grow, even though it is just about the only remaining subscriber-based game left. They just have to stop giving boneheads free rein, and when the subscriber numbers stop rising and go off a cliff, figure out what you did wrong, and FIX it! Stop doubling down on failure. It has not worked for the past five years, and it won't work now.