People can place blame on the loss in subscriptions due to the age of the game, but I firmly believe that Blizzard's inexorable campaign to dilute the complexity from this game and democratize the content are both big factors. They feel that everyone should be able to experience everything within the game, and that if something is too "difficult", then it requires an overhaul.
I'd look into the history of Lord of the Rings Online to counteract that argument a bit. Maybe it was a different audience, but that game was a mid-level game, certainly not easy, certainly not harsh, that was growing. As soon as they started adding end game raids it got a little better. Then they started GATING end game raids through gear that took hundreds of hours to get. And such was the end of the game, starting with Mines of Moria.
They started losing subscribers, and they lost enough to follow up Moria with an expansion that shifted towards FtP, and democratized content a little more Then, I believe 24 months after Moria they were on the FtP kick to reinvigorate subscribers, which also didn't work.
They were/are done pretty much. It's still kick with sub 1-million players, but that's about it.
At its prime, the game was a more structured EQ with a solid, challenging end-game. Of course, the big guilds could farm end game stuff, which did screw the market, but that was it. They made it harder and made gating time consuming and it completely failed.
This post was edited on 8/6 at 1:08 pm