The idea that AA is only supposed to suppress planes and not kill them just seems extremely silly to me.
Imagine if devs decided that anti tank guns were not supposed to kill tanks and only to supress them and tanks should kill tanks.
A plane can only be countered by two types of units - AA and other planes and the decision that aa is only meant to supress is something i dont think i will be ever able to agree.
Try flying over the 3rd Armoured Division, nearly everything has a machine gun on it and it's like a fireworks display when they all open up. Seeing as this drives off planes fairly well I would say that groundfire can also be a counter.