I think it takes an $800 computer setup, a $35 joystick, and $15 headphones -- not that much -- and you can have a very good time.
It doesn't take that long to become competent at bombing (not a gunning ace, but competent in bombing) or GV combat (not a tank ace, but competent at fighting). It takes a little longer to get competent at divebombing. Yes, it takes much longer to get competent at air-to-air combat, but that gives you levels of the game into which you can grow. It takes a long time to get up to higher levels in World of Warcraft or to become thoroughly integrated into EVE Online, too.
Yes, a flight combat sim isn't going to have the same potential audience size as WoW, but it doesn't need that to be successful and lots of fun for the players. I think it's all about player density, not player numbers.
What is a sufficient amount for fun? I would say that as long as a given player has about 5-10 other players around him fighting, it is fun. Air Warrior was fun with 10 people in the whole arena -- but AW arenas were two AH sectors by two AH sectors in size.
So, the game could be fun with 5 in one country and 5 in the other as long as those 5 are fighting it out against each other.
What could help are maps that get smaller, or some aspect of the map that funnels participation into a smaller area, when there are fewer players.
This, to me, seems to be the key.
All of this lamentation about GV's or even about player numbers is missing the vital thing: density.