This is the biggest problem with the idea (another one is that it will cause more issues for colorblind players.) We already have a lot of ambiguous icons that don't do a player a lot of good until they know what they mean (Concussion Shield/Damage Resistance, anyone?)
I'd like to see the old "population chart" make a comeback, the one that was above your minimap, not the one that you have to open the map to see. Then, instead of using player populations, you base it on the overall military strength. More force multipliers in the air/on the ground = more bar. Enemies bar = bigger than yours = it's time to do something new.
It's not just dying to air that's frustrating, it's also armor, and max units. A clear cut, "You don't have enough strength in this area to do anything useful" visualized in bar form would kill many birds with one stone. Poor birds.