So lets look at WoW's history:
Vanilla WoW - built up and ended with 8 million subscribers.
Burning Crusade - Built up and ended with 11 million subscribers.
Wrath of the Lich King - Built up and ended with 12 million subscribers.
Cataclysm - Lost 2.9 million subscribers ended with 9.1 million subscribers.
Mists of Pandaria - Lost 2.3 Million subscribers, ended with 7.1 million subscribers.
Warlords of Draenor - Built up then lost 2.9 million subscribers, ended with 7.1 million subscribers again (All in a span of 3 months)
The losses began when Activision took over Blizzard. The story telling took a nose dive. The quality of the expansions took a nose dive. The gameplay itself took a nose dive. People tried giving WoD a chance, then said "Nope enough of that" and left again. Content is indeed at the heart of the problem, but it isn't "lack of content" it's the quality of the content. It's the backpedaling Blizzard did on certain things. Example "OH don't worry! Our in game store will never sell character effecting items that give stats, experience or levels!" then a few months later "Hey guys! For the price of three expansion packs you can be level 90!"
Cataclysm was the start of the decline. Female worgen looked like chihuahua prostitutes, when they should of looked like werewolves with breasts. Zone after zone, was nothing but easter eggs, with no original story telling what so ever. Westfall - CSI: Miami. Redridge Mountains - Rambo. Uldum - Indiana Jones. This list goes on and on. Then came mists of pandaria. Cross realm zones was a huge contributer to it's loss. As was it's story telling. Cross Realm Zones, was extremely buggy, and in many cases, when crossing a zone border, it would freeze people up, and once the lockup ended, the character would burst out in a bright flash of light. That was giving people, myself included, headaches, and very well could cause seizures. It broke old world events, and made passenger mounts people invested time and in many cases, money, completely useless as it would kick passengers off upon crossing a zone border. Especially if it was a flying passenger mount, the passenger would fall to their death.
The story telling got worse as the expansion went on, and even the website stories, were bad. Like the one where a baby was killed for absolutely no reason what so ever. They attempted shock value, but all it did was disgust me, because really, killing the baby did absolutely nothing for the story.
Then came Warlords of Draenor. Even worse story telling before it even shipped. Throws away all the lore we've had up till now, and gameplay wise, people were bored within a week or two of level capping. But Blizzard's solution isn't to admit mistakes were made, and instead they think their players brainless dolts who will buy anything with their logo on it. So they are announcing a new expansion in hopes of getting 10 million again. It's time for that game to just retire. But the heads of activision-blizzard, either didn't get the memo, or are ignoring the memo. Like WoD, I have 0 interest in another WoW expansion. I didn't buy WoD. But many of my friends did, and were, on the first week it was out saying "Oh it's so much better!" and I told them "Uh huh. I will see you on FFXIV next week.". Those same friends in less than a week, were back on FFXIV saying they wish they never got that expansion because they got bored fast.
These AAA developers need to realize. When players find flaws in the quality of their product, and subsequent product, the customers eventually do learn their lesson and stop buying subsequent future products. They only hurt their own brand. Activision-Blizzard already damaged their brand with now former long-time fans. Buuut there is one positive thing I can say about Activision-Blizzard. They aren't as bad as Electronic Arts!