So you've never flown over an airfield on your way to hit a town? I know I have.
As far as them flying too high, surely you know that an 88 can reach quite far. With a trained gunner (assuming this is manned) the results will give many the bomber pilot cause for concern.
I have too, but if the airfield would become dangerous, I would simply avoid it.
And it's not because the 88 can't technically reach that far. It's because nobody will hit. Even the 5" manned guns on CVs have never ever managed to shoot me down at >20k. A typical town milkrunner altitude is way above that. A formation of bombers, flying perpendicular to the gunner in a distance of 1-2 miles at an unknown altitude (but somewhere around 25k)... that's an almost impossible target even if the gun had proxy fuses. It's just 1-3 very small dots without any indication of actual distance/altitude.
Sorry, but with the information we have now, it's in no way at all discouraging the proverbial 'town milker'. I certainly will be not. There would have to be a lot of additional changes to do so. They may come, but as for now...