It is correct that a unit get experienced and battle hardened after surviving combat, and it reach the peak after 100 hours of combat, then flat out to 400 hours and after that the fighting spirit decline because of combat fatigue and exhaust, not to mention most of the men got traumatized. A good example is the Desert Rats of 8th Army, they knocked Rommel out of Africa and fought as lions during El Alamein, but in Normandy some years later they were old men worn out and the kids in Hitlerjugend had no problem pushing them back.
So to keep this rule rational, after 3 or 4 battles the veteran unit is again promoted, but this time back to a retirement unit with 0 as combat value.
Not to forget that any specific army unit would keep getting fresh replacements of unexperienced recruits to keep up the steady loss of combat hardened veterans. So I say lets keep the units at the current values. Besides, modern weapons, high fighting spirit and morale, good training and skilled leaders, and not to forget a working supply line, have way more impact on the units combat effectiveness than some veterans that accidentally survived a battle and got battle hardened.