In the T1/T2/T3 stage, air serves a number of useful purposes: scouting, transportation, bombing weakly defended targets, high-mobility gunships able to chase down raiders. And interceptors.
In the late T3/T4 stage, one team tends to build a huge ball of ASF able to shut down the opponent's air, even over the opponent's mobile land armies. Worse, if one team gets air supremacy and 20+ T3 gunships or a T4 gunship, they can shut down most land pushes.
This changes the game to the point that land pushes are hard, turning the game into a T4 arty war.
Suggestion: add a T4 mobile land AA unit to each faction, somewhere around the cost of a spider or GC. Make it strong enough to beat all T4 air and able to kill massed gunships quickly using AOE weapons.
This should make multiple-T4-pushes more effective and reduce the incentive to make 100+ ASF.
A simpler alternative would be to buff T3 MAA, but I think this has more downsides.