I notice you put '....' instead of 'Done properly'.... done properly, AA wouldn't lose their place in the battlefield, like it seems you are suggesting. How much lower does 'Mech AA capability have to be? Some don't have any AA capability at all, (close range variants, I'm looking at you), and many, long range variants or not, either can't look up high enough, or the cockpit view is obstructing enough your reticle is just above the line between glass and metal, you can't see anything, and you're frantically swinging the reticle around the red box waiting for it to turn red. That's assuming your weapons don't go either side of the flying asset, or cross (X) in front of it and miss it anyway, depending on weapon location. Thats also assuming you're even able to keep the guy in your field of aim, and he hasn't just flown up and/or over you.