You are missing the point entirely. Airships can fly. They can fly high. So high anti-air guns start to be ineffective. In this case, one could build ships specifically made to pretty much carry anti-air guns up with them for when airships are out of range of ground emplacements. I am not saying everyone should make such ships. I am saying you could. You could also try to develop ground emplacements with better range. But if you want mobility, the ships are a good option.You don't need to fly to fire at the heavens.All you have to do is put wheels, or rails on it and the AA suddenly has an incredibly vast range.Wheels don't make you fly.