Tall Paul,
I understand what you are saying and I do consider airborne to be a type of “Elite/Special” unit. In our games, airborne units HAVE to be taken by transport planes AND only 1 paratroop is allowed per transport plane. So, for each paratroop drop you are looking at an investment of 13 IPCs each (4 IPCs = paratroop, 9 IPCs = transport plane).
So, even in the case of the US that starts with 52 IPCs, if they wanted to spend their entire IPCs on paratroops, they could only get 4 paratroops plus 4 transport planes.
In effect, the requirement to purchase a transport with each paratrooper places a limit on the number of paratroops any nation can have on the board at one time.
Granted, I guess a nation could keep buying transports and paratroops over several rounds resulting in one country having a whole bunch of paratroops. However, that would probably be an unwise strategy for any of the nations to buy paratroops and transports only. No tanks for Germany? No combat aircraft for UK? No warships for US or Japan?
I think the other nations would welcome such a development. “Oh, you have a bunch of paratroops and transport planes? Good! Now I’m going to invade you with my stack of tanks and mechs.”
Now, perhaps you don’t use the transport aircraft and simply have the airborne units launch themselves from air bases (like the OOB tech rules)? In that case, I could understand a rule limiting airborne units to a certain number like with any other “Special/Elite” units.
By the way, do you use other elite units like SS, Guards, Marines, etc.?