I don't think the Battlefield games lure players away from Aces High. They are two different genres. War Thunder, WW2OL on the other hand...
I believe they do.
I think there were a LOT of Aces High players in 1999-2005 that were perfectly happy to move to BF once it became available. Where do you think the 700 players we used to have all went to? Not just to BF, but I bet many did. If not BF, the WT, or WT, or WoW, or WIIOL, etc.
A lot just wanted online multiplayer combat. They may have been OK with airplanes, but a lot didn't necessarily eat, live, and breath WWII fighter aircraft like the old timers you have left here now. They were like, whatever, airplanes, yeah I can do that for a while. AS other new mutliplayer online combat experiences came online they got lured off a few at a time to other games, even different genre.
Back in 1999, AH was really the best, if not the only, option out there for online multiplayer combat. Now players are drowning in options. With multiple options of monitization.