Haven't you thought that in such a scenario, your 10+ units that you call a "pain in the ass" will start to be placed at every step? And with your 50%, the game will turn not into capturing territories, but into "campers", who will be waiting for an attack on them, sitting in a cocoon.
This mechanic destroys the balance and creates a precedent. That is, besides the fact that your fortification already raises the defense % to 90 percent for the defenders, it will also have 50% of personal defense. Calculate mathematically what it will take in time to destroy such a fortification, and the number of units you will lose as an attacker. After all, your bombers will be hitting a fort that gives 90% defense to the units inside, and itself absorbs 50% of damage. At the same time, the AA inside the shelter chews you with bonus damage from morale, from defense, from terrain. Are you yourself such an idiot to attack such a fortress? I am not. 90% defense is already a pain in the ass, which in the hands of a skilled player brings you losses during such an attack on him.
