in the U.S. Army, there's a thing called a TOE or MTOE. I wouldn't doubt other military forces have a similar system. logistics, basically.
a concept of this though is already in War Thunder (I can talk about other games here, right?). basically, you do earn mods for most aircraft, if they're not fully modded, and that some of them have repair and ammo costs.
so, every time you land at a strip, or after you leave a fight, repairs and ammo replenishment can be done "on the fly" (if you allowed the settings; no pun intended) - sometimes its free, sometimes it's not. the cost of that is way cheaper than buying crews and new aircraft and training. not to mention, it's negligible for the fact that you are "paid" for each fight, and sometimes get more bonuses for every surface unit destroyed, or downed enemy aircraft.