The solution is not looking at how to retain it is in looking at how to draw.
In my humble opinion it is too late for AH to draw in any significant amount of new players without being both free to play and upgraded considerably in the graphics department.
Sad to say I know...but I am convinced it's true.
All of the great things that AH has going for it count for nothing when potential new players see something like War Thunder, which may be an inferior game, but is free and pretty.
That's without even mentioning the steep learning curve of AH which makes it really tough for all but the most dedicated to get satisfaction from the air combat part of the game.
War Thunder is free, EASY, and graphically stunning. AH simply can't compete with that when it comes to drawing in new players, in the long run.
So it's my opinion that we have to look at ways of retaining players, instead of believing that it's possible to make some kind of magical comeback and gain a lot more.
HTC's ignoring of the problem of low player population during Euro + Pacific prime times for the last year or so has just led for most of us in those time zones to unsubscribe.