The problem lies in attack, not in defense (although defense is easy with planes too).
As I understand it, air units are lethal once again. I admit that I didn't think about the resource requirement (old habits die hard
), but you could still stack maybe 20-30 planes in a single city if you have a large enough empire. Interception is hardly a problem against AIs I would think (ofc this remains to be seen; traditionally the AI has never understood the importance of air power).
So assuming you have similar numbers on the ground, you could double the size of your army and give it extra reach beyond the enemy front lines... If my ground army is all cavalry, I could kill one third of the enemy units with my planes and weaken the rest, then mop up with the cav. Hammer cost to me (from intercepted planes) maybe a few hundred, thousand+ for the enemy. Come back when I take your city without losses (a few planes at most) and tell me that's not unbalanced!
If the AI understands the concept of concentrated force and can react against stacking planes (by stacks of its own... Remind you of something?
), then my concerns may be unwarranted. However, this is a difficult thing to balance: if the 'waryness' algorithm is too wary, you could lure the AI's troops away with a concentration of obsolete units and then attack from another direction with your effective main force. The other extreme is that the AI turtles in its cities, as in previous civs. Or well,
near its cities, since it can no longer stack its troops in them.