It seems a lot of people around here just assume that there’s a massive group of gamers who WANT to play the same game for 5, 10, 15, 20, etc years. That’s what the MMO market demands, lifelong subscribers.
If anything, WoW was a flash in the pan, bringing in a huge number of people early on due to the popularity of their IP and a relatively low learning curve. The decline after WotLK (I think) is less indicative of a decline in the quality of the game, but rather the threshold that most gamers are willing to dedicate to a single game (1-5yrs).
The longer the game goes, the more people will drop off until you’re left with only the people willing to play WoW for the rest of their lives. I know I’m not one of those people. 15yrs is already pushing it, I’m not excited about SL, and I’m not even one of those people who think WoW has greatly declined over the last decade.