I have to agree with the others that air is fine the way it is now. Yes, your planes can be shot down by a 3 cost INF, but so what? If you are crazy enough to send in planes without the fodder to ensure their survival, they should die.

It is also very similar in a historical sense. When planes are making ground attacks, sometimes they would come in low, 40-150 ft off the ground and straffe the battle. This was done because is was often not practicle to drop bombs from fighters on large groups on infintry and calling in bombers is a bit pointless because the INF woudl more to quickly for the bombers.

This being said, with planes flying that low overhead, it is very possible that and INF or tank mounted gun could shoot them done.