- VG are badly design, they can fight at all ranges and deal with most situation cost effectively, leaving little room for other unit.
- The reason why OKW infatry is powerful is of little importance as long as they are allied infatry will have to remain strong
- VG's performance effect both balance and diversity issues, an does penals performance
- One has to start nerfing somewhere and VG/Penals is a very good place to start
Volks get pummeled by Penals, upgraded Riflemen and bolstered Infantry Sections. It's the reason OKW is so reliant on light vehicle spam into Command Panther. The fact that they can fight at all ranges isn't always an advantage because it also means that they're not good at any range - and lose to Allied infantry whenever those are in their preferred range.
The reasons why OKW's infantry composition is powerful is the most important of all, because the biggest reason doesn't have to do much with Volksgrenadiers' raw performance but much more with OKW's ability to flood the field with a very strong starter unit backed up by good cost effective infantry in very little time, giving them early map control. Tone down OKW's starting manpower (and give them back some power in the mid game with earlier Obers and scalable Sturmpioneers) and the Allies have a much better chance to get into the early game without having to nerf any unit's performance (which would probably have more and bigger indirect consequences).
Assuming you mean current OKW early power (caused by Volks performance according to you) causes diversity issues in Allied build orders as in they are forced into certain units to deal with them, again this would also be solved by toning down OKW's starting resources. The problem is not that Riflemen get outperformed by Volksgrenadiers, over time they won't (Rifles are equal to better at minute 0 but pull ahead once they get vet and upgrades), but that they get pushed off the field early by the Volks/Sturms combo. Tone down the speed with which OKW is able to flood the field, and you'll solve the early game balance problems without also ruining the mid game balance (by nerfing Volks's raw performance) which is mostly even between Volks/Penals/RM/Grens.
I agree that Penals are a little bit too strong at minute 0, but Volksgrenadiers are not. However Penals could only get nerfed after OKW receives some early/mid power changes or the Soviets would get wiped off the field as Penals + M3 is the only way for the Soviets to reign in OKW's early game at the moment.