So let me get straight, I have a tank division with Panzzer 4As, and they should never ever upgrade at all until the Panther is designed? We justify this stance in the name of historical accuracy? Somehow you haven't sold me here.
I am not challenging the system when it works

. I am challenging where it doesn't.
I fully agree that this system works beautifully when it comes to a "smooth lineage" of models like the Pz-IV line or T-34 or Sherman "dynasties".
My question is what happens at the "fracture zones"?
A Pz-IV can only be upgraded so far just as a T-34 and a Sherman tank. They all have a limit. The limit is 9 times of 10 the inability to mount a larger gun.
Eventually a new design that denominates a completely new generation rolls off.
One such "fracture zone" is the transition from the Pz-IVF/G to the Panther tank. This is not an incremental step forward,it is a downright jump. New armor,new gun,new engine,new everything.
Or another "fracture zone", this time in the heavy tank field is the jump from the KV-1 (model '43) to the IS-1. New chassis,new engine,new gun,the works.
Or the Churchill tank. The Churchill wasn't a "modified Matilda" just like the KV-1 wasn't a modified T-35 or Panther isn't a modified Pz-IV. It was a new design.
Shouldn't there be two systems working in tandem?
1) An incremental system that signifies the improvements brought upon an established generation.
This means recognizing that a given chassis has a set "improvement potential" that is finite and well defined and that there is a technical limit beyond which a given chassis cannot be improved upon.
The most important limit should,of course,be the gun type (large caliber,long barrel).
2) A "jump" system that signifies the advent of an entirely new generation of tanks different clearly from the previous one.
This should allow players to really bring out shiny new toys,not simply "patch up" the old ones.
Referring to the situation you have brought up, a "step by step" system correctly describes the progression from the Pz-IV A to G. But this system does not correctly portray the qualitative jump from the Pz-IVG to the Pz-V Panther.
I would recommend that a new component be added,one that "cues" an across the board improvement :
chassis . Different from "armor". One that also gives the main name of a given model.
A T-34 is the chassis.Not the armor or gun or engine which can all be modified.
The chassis should be the "core" of a model. The chassis can be up-armored,up-gunned,fitted with a new,better engine (err...where are the tank gun sights? they are present for indirect fire artillery but absent for tanks?),better reliability, it can modified for specialist vehicles (SP-AT,SP-ART,SP-AA), up to a limit of course.
A new chassis should automatically include the latest gun,armor,engine (reliability can reasonably be left out) in the upgrade queue (with the adequate costs and all).
This is one universal rule, that the advent of a new generation of tanks includes the best available at the moment,doubly so for armament.
Thus, the issue that I am raising is that there were two types of progress :
1) Step-by-step improvements of existing models (Pz-IV A - F, T-34 models,etc)
2) Completely new generations clearly different from the previous ones. The examples are several,I've enumerated some before and here are some more :
-Transition from M5 Stuart to M24 Chaffee
-Transition from Matilda I to Matilda II (it is a common misconception that the two Matildas were related,but they were not.)
-Transition from the M3 Lee to the M4 Sherman (The M4 Sherman was not an improved M3,the M3 was an improved M2)
-Transition from the Crusader tank to the Cromwell cruiser tank