I think a lot of it has to do with the fact that the average player now, in 2009, has become the younger, video game bred player who worries more about score and "winning" than the average player of 5 years or more ago, who was generally an aviation and history enthusiast here to get a bit of "the experience".
I was never in AW, but I've heard from several who played it that they really started going downhill when it was released as a boxed game and kids with no patience or knowledge of what the game was about started playing it and drowning out the "old breed". So many of them went over to WB.
Now I played WarBirds for several years, and was asked to become a CM just before I left it. I got fed up with it because of the almost daily swing from a simulation to a game, with the player base becoming more and more of a war-winning horde than a group of enthusiasts enjoying the fight and the experience of the scenarios and events. So I left in 2001.
When I came here and began flying regularly in 2004-05, the game was like WB was when I started- relatively mature group of core players and good, intense fights with folks who were in it for the fun, not the score. Then the massive influx of kids started, and it almost overnight got much worse than WB ever was. I've several times taken long breaks, usually with the intention of not coming back. Eventually I get bored with boxed games and come back for a while, and every time the progress is apparent on just about the first sortie. First was teh arena split, then the larger and larger maps, and finally this "Titanic Tuesday" which seems like an online daycare service instead of a flight sim. Now I usually spend half my time chasing high cherry pickers around the map who refuse to fight, even when they have every advantage, and the other half fighting mass hordes of skilless score-monger clones. Even among friendlies you can see the gameplay dropping fast- in the old days, you would be asked if you needed help before someone jumped into a 1v1, now you have to get to ramming distance to keep from killshooting yourself on all the kill stealers.
I can see why the "vets" are starting to leave in droves, and maybe they'll be coming back when they get bored and start to miss the good times. Unfortunately the "good times" are now far outweighed by the increasingly lame style of gameplay and immaturity of the players- some long time players who've been "infected" included.
I don't consider myself an "old timer", but I've gotten to the point where I basically just stick around for events and scenarios. I usually only go into the MAs or into the AvA during regular hours when I'm looking to try out a new plane or get used to a new setup in "combat" instead of offline where you can't really try it out.