All very good. Thanks for all the replies
I will try to respond by just reiterating the whole Air Power thing
1) Dogfigthing Fighters: HLC (which is probably anything level 8+) supremacy can be devastating, but because of this, HLCs spend a lot of time battling each other, and this could absorb most of their time and resources.
2) Control of the sky brings many benefits: basically I am saying that Air Power is the best "modern analogy" for HLCs, better then tanks, artillery, etc (which might be good for mid or low level charecters) in part because of the wide raning impact. Control of the air adds airborn mobility and intelligence, not just firepower (and yes, finding and hiding aircraft has been a big part of the battle for air supremacy for decades). But it is true that magic is magic, and by its nature, you can do all kinds of things with it.
3) Airplanes cost a lot, but it may be a cost worth paying: There is little question that higher level charecters--even mid level charecters--are relatively rare and expensive. Though it really depends on the campaign on how rare or expensive. A kingdom could have one 14+ level HLC, or over a dozen, who would be willing to fight for it. But it probably doesn't have hundreds (though I guess in a planscape type campaing that would be possible).
3A) Based on RL, a small kingdom or city state can mobilise thousands if needed. A large kingdom or empire ten of thousands, including thousands of elite troops (highly trained longbowman, knights in full plate, Roman Legionaires). And historically, there have been hordes of 50,000, and larger! Atilla the Hun had 100,000 at one point. Of course, there can be smaller battles, but then the HLCs might be 8th or 9th level, not 15th.
4) Focus on high value targets: the Air Force loves to take out command centers, and hates to go after cheap soviet era tanks (though they can). When HLCs aren't going after other HLCs, then there are the seige engines, the elite troops, the medium charecters. Taking out 10 commoners with a fireball (fired foward at loose formation) is a real waste.
5) Aircraft can be shot down This is probably one of the main points of contention. The improved invisible wizard comes in and starts throwing spells around (stealth bomber), what countermeasures are there? My guess is that there is probably something, maybe invovling an adept with a wand of detect magic and crossbowman with enchanted bolts. Or the barbarian with the spiked chain, again, my guess there is something (charging heavy cavalry with lances?)
ok, keep discusing!