I dont agree that a gunship unit of strength 1 should represent severely damaged gunships. The unit represents a military formation (division, platoon, whatever, depending on the scale of the game/scenario). So a unit with strength 1/20 would probably mean 1 in every 20 helicopters is operational, rather than all the helicopters are grounded due to damage.
Further on, real air units are impervious to almost all ground units. You cant damage a fighter or bomber with a tank any more than you can damage it with a spearman, and this is represented in the game. On the other hand, air units cannot conquer territory. They can bomb the hell out of a unit of musketmen, but some of then will still hide in caves, or forests, or bunkers. The unit of musketmen cannot fight back, but it also cannot be truly destroyed, nor the land it is defending taken. You need ground troops to do that.
Gunships represent a middle option. They are, in effect, highly mobile ground troops, flying at very low altitudes and supported by helicopter-carried troops that secure the ground after each successful strike. If the enemy is cunning enough, or entrenched enough, they can thwart the attack of the air cavalry by evading the gunships direct attacks, ambushing the ground troops, and perhaps launching surprise counter-attacks against the helicopters bases of operation, leaving them without fuel and support. That would amount to destroying the gunship unit with axemen.
And of course, even if a units main weapon is the axe or spear, in modern times you should expect them to make smaller use of more unconventional weapons, suited to each unit they confront. Spearmen equipped with Molotov cocktails (a cheap trick that would not show on the official unit stats) could take out early tanks.
Therefore, I personally find the current system of battles satisfactory.