From my understanding most of it goes back to EA's money grubbing rather than the fault of the game, or the setting, or WOW, or anything else. No new material, no money to fix the growing bug problems, hell entire parts of the world (Ulthuan) were abandoned towards the end as they couldn't bother to fix them.
Unfortunately Mythic was bought out by EA and we all know what EA is like, when it didn't line their pockets as much as they would like it to (they probably thought it was going to profit them like WOW was for Blizzard) they abandoned the game for the most part. You do not even need to be the next WOW to succeed and stay alive, just get enough player base to be successful and keep going. I don't understand why it is that every time someone mentions MMO someone has to say it in some way has to challenge or replace WOW? Final Fantasy XIV has been doing pretty well after it's reboot and expansions and it can do this without having to supplant WOW.
It's hard to keep an MMO alive and no MMO is able to pose much of a challenge to WOW anyway.