It has to do with the overall durability of a unit. In Relics reasoning, larger squads are more survivable:...
More entities (at same HP) make squad more survivable but lets leave AOE weapon out of this since they are irrelevant.
(PPSH) making it in fact a side grade and not an upgrade.
Is currently one of the most cost efficient upgrade in the game...
You haven't yet proven why you can't compare doctrinal upgrades to nondoctrinal ones but I'll bite: M1919s are doctrinal and 70 muni a piece even though they perform identical to MG42s.
You start good but then you go and compare the doctrinal M1919s with stock LMG42...
Using the logic you want me to follow Fusilier G43s should cost drastically more than Grenadier G43s because...
It already 33% more expensive although it is 8.8 less effective. so its actually around 40% more expensive at the price of 35 it would be 55% more expensive and more 64% taking into account the difference in DPS.
It would also be extremely more expensive than the Ostheer ST44 at 25 MU...
If you compare the DPS of squad you can get a better picture:
(m1_garand_rifle_rifleman_mp)x5---------33.81 21.67 14.03 9.89 8.37
(panzerfusilier_kar_98k_rifle_mp)x6-----22.58 18.45 15.46 12.53 11.08
(panzerfusilier_g43)x3+3-----------------38.63 29.49 18.31 13.35 11.52