Disengagement is the art of withdrawing successfully from combat, normally done so at platoon and lower levels, but under divisional terms, its known as a 'fighting withdrawal'
This skill, which is one only few generals ever master, is extremely hard to pull off. Many generals do send men into battle just to die, this a soviet tactic- "bodies for time" as the Stavka used to say. CoW uses this 'Bodies for time' theory rather well, but if your army has Artillery and is being advanced on, alot of people leave the arty to die and pull back the rest of the troops- or vice versa. Either way, a 'retreat' system is highly debated.
Combat experience has been put forward before, but due to CoW unit structure, it becomes pointless for a point you've already mentioned: you just build new units, and two- unit tech is replaced.
For example, lets say my 1st Light Tank brigade is the crack tank brigade of the German army, its well rounded, has lots of experience points and has served well. Soon, I get Medium tanks: which are harder hitting, take more damage and generally more useful. Why would I want to keep using lesser tech just because its experienced? I'd be a fool.
Generally speaking, real life combat units do get a level of experience, but its usually for more unorthodox things. As the son of a soldier, and someone who spent his life around soldiers, you don't magically learn new skills in on battlefield: you learn how to put your clothes on lying down and how to keep your food dry and sneak extra stuff. Hell, I was taught how to increase the weight of my back-pack for training exercises without actually adding anything heavy.
