So what is the solution than, or is there one? With attrition + combat losses+ further and further from supply the front line infantry divisions deplete very quickly and without those merges I will soon (ie 2 or 3 turns without doing this) have very few units capable of attacking, particularly in those difficult to supply areas. Not saying you are not right, I am burning through a lot of APs to build more brigades, just don't know what the answer is. I suppose I could turn my TOE down to 50% on all units not engaged in combat, in partucular the newly arrived units that need time to train up anyway. But will the replacements reach the front line units anyway? Another possibility is to rotate units in and out of the front lines. But this is difficult when the front lines are in places 2 or 3 turns marching distance from the nearest railheads.
Largely the nature of the beast that is the Russian blizzard offensive. What I try to do, is set my attacking armies to refit, so that they get priority for repalcements. Typically one has to mass enough units in areas to get the offensive going, so that you aren't getting all units into the line all at once anyways. Armies thus end up attacking in echelon: Army X pushing ahead for a couple turns with Army Y following (on refit building strength). When Army X starts losing strength, Army Y takes over giving Army X a couple turns of rest and rebuild on refit. Alternatively, only half (or so) of each Army X and Y are attacking each turn, leaving the other half to rest, rebuild, and catch up.
You can't attack the way the Germans do. You're pumping large numbers of men (and large casualties) into the battle, but that's just a good old fashioned meat-grinder of the proletariate!