I personally have not a problem with the current default experiences. And what is said just above is truly correct.
Germans in general fared very well in the war for the numbers they had. When Allied started to net their victories, they had such an overwhelming advantage in terms of materials, resources, and manpower that defeats were pratically impossible. Yet many of their battles were phyrric victories or the like. From a Sicily, to an El Alamein (Where, even despite fuel shortage, the Afrika Korps slipt away to fight another day in Tunisia) and further on as the Allied economical superiority turned more and more prevalent.
But then - talking of WarPlan - late war the German manpower will pummel so their units will be similar to Allied ones.
A treshold of replacements where there is no experience loss would be way better suited for the game - unit by unit. My line of thoughts. (Ie, unit has suffered a few hits, once replacements kick in they're not bringing the whole unit experience down. If they got a hammering and the unit is half strength, then yes, that massive influx of replacements will dilute the combat experience of the troops).
That way veteran troops emerge from long campaigns - at some point the fate of war may get them hammered or even destroyed.
Once grim days and times arrive, any new unit will have a low experience, and so replacements too. That is good.
But right now there is no real curve of improvement in the early days where things go nice and smooth as usually the combat dones erase most of the experience accumulated, and the units for Germany bounce up and down between 70 and 75.