For aircraft you would want a different turret than the tank has, and would have no need for the MBT's armor. You can certainly make a short range air defense vehicle using a chaingun, though that shouldn't be its only armament.
For infantry bigger explosives suppress better than just bullets. So a 120mm gun can do a lot of suppression and destruction than a chain gun could. Plus the tank should have a couple of machine guns on it for engaging infantry if the main gun doesn't work for some reason. Plus it is more versatile.