The only squads where upgrades are not mandatory at some point are squads that have powerful or specialist weapons stock.
Conscripts went for a long time without stock upgrades and suffered for it. The only thing that allowed soviet infantry to stay competitive was doctrinal elite infantry and penals. That's despite the theoretical "utility" cons provided. Take away the 7 man upgrade and cons will be rarely taken without doctrinal upgrades just like they used to be.
The power dynamic between the stg44 and allied infantry is a good one. Volks trade poorly with rifles initially, situationally poorly against IS, and evenly against conscripts. Volks recieve a potential power spike if you havent been dumping tons of munitions into grenades. Allied factions recieve a power spike shortly following this via racks or in the case of Soviets, elite infantry. This dynamic is very important as it forces the OKW player to weigh the costs of throwing tons of low damage grenades early on. UKF and USF dont have repeating munitions costs early on(unless you go for grenade tech) which let's them bank for rack upgrades or medkits. Soviets is a different story, but this timing and munitions expenditure dynamic is made possible by the ebb and flow of infantry power. Taking away volks power spike will disrupt that, and I have not seen a good reason that should be the case except that you find the upgrade "boring"
If you feel the upgrade is UNBALANCED, then that is another story.
You know what else is a boring upgrade? Pintle machine guns. Or rear echelon sweepers. Or cavalry rifle thompsons. Or UKF tank commanders. Sometimes boring upgrades are necessary to allow units to reach desired performance levels without being free.
You may as well raise volks cost to 300 and reinforce to 27 but give them a stat increase roughly equivalent to the stg upgrade. At least that way they'll scale properly and have a similar investment in cost over time while not having a "brainless upgrade".
Edit: I'm unsure as to why you are referring to volks as "durable". Once upon a time they had a recieved accuracy of like 0.6, but those days are long past us. In fact they are the second squishiest stock mainline infantry after grenadiers, even after vet. With vet, they match a Vetted rear echelon for durability.