Here's the quote about modern vs older aircraft:
Just remember that our budget is limited to about 3 new player planes (or 1 modern plane) and similar number of new AI planes per game. And that number is even less if we also have to do a lot of new ground objects. And also remember that add-ons generally sell less, so we can include even less new content in them.
So for example, if we were to do Korean War next (which we we're not doing) we'd have to do a limited scope version first and probably only do F-51, F-86, and MiG-15. Or, since P-51 and F-86 and MiG-15 are also used elsewhere, we can wait until they're done, then do a slightly bigger Korea later that also includes Skyraider and F9F, etc etc.