You note that the side with air superiority usually wins battles, and seem to conflate that to mean that being in the air is universally better than being on the ground, which is not the case.
In real-world combat, air forces are what's known as a "force multiplier", which is basically to say that they make existing forces more effective. However, as anyone with grade-school knowledge of math can tell you, multiplying 0 by anything still results in 0, so while having air superiority is an important factor, it's not the be all and end all, and there are many wars that speak to that experience. The Allies had air superiority over the Axis powers for years, but it took troops on the ground to win that war. The Soviets had air superiority in Afghanistan and lost. The Americans had air superiority in Vietnam and lost. Etc.
Examining these wars (and even more recent conflicts) reveals a couple important realities about aerial combat.
The first is that you need ground troops. You can't simply bomb an enemy into submission from the air. It's been tried (a lot), without success. So as important as air superiority is, it's not enough on its own.
The second is that things in the air are easy to spot and target, and hard to defend. They can be seen for miles, can't be heavily armored or they're too heavy to fly, and have to rely on maneuverability to avoid attacks, which is why you see multi-million dollar helicopters being shot down by untrained child soldiers with a $100 RPG, in the real world.
Your world is a little different, but the same principles would apply.
In the context of a world with bows and arrows and swords (as opposed to SAMs and automatic rifles), the aerial advantage would probably be not as great as it is today, so I don't think it will be as unbalancing as you think. The biggest advantage of a modern-day air strike is the ability to bomb an enemy position from a relatively safe position (a jet zipping by out of range of ground forces). We don't have any real-world examples of flying soldiers armed with bows and arrows, but I have to believe that if they were out of range of enemy archers on the ground, they'd be pretty ineffective, and if they got close enough to be especially effective, they'd be in danger of taking effective fire themselves. It still sounds like an advantage, just not an overwhelming one.
Having said that, as in the real world, your angels wouldn't be able to achieve victory by simply flying over the enemy and raining down aerial attacks anymore than we can, so they're going to need to put boots on the ground at some point.
Secondly, if your angels want to wear heavy enough armor to deflect the ranged attacks of the demon hordes, they'll be too heavy to fly. Or at the very least, they can't armor their wings and still fly, and if they were to lose their ability to fly mid-flight, the fall would kill them. Hence, the angels have a trade-off to consider - good armor and no flight, or flight, but with ineffective armor. Especially facing hordes of enemies that could shoot them down with a hail of arrows, this makes flight dangerous.
Ultimately, your angels would probably (or at least should) come to the same solution we come to in the real world - mixed forces (both air and ground) coordinating their battle plans for maximum effectiveness. They wouldn't choose one or they other, they'd choose some combination of flying and ground forces, just as we do.
Why would an army of Americans with air superiority fight Vietnamese/Iraqis/Afghanis/etc. on the ground?We know they do, so there must be a reason why we see this dynamic in the real world too. $\endgroup$