If you check Volks veterancy bonuses you will see that at Vet5 the are much stronger than Conscripts and Rifles without any upgrades. There goes your theory about Volks being weak. They don't get any weapon upgrades because OKW have access to super soldiers in the form of Obersoldaten.
The point is that at vet 0, they lose out to every single mainline allied infantry unit. At vet 5
any OKW unit is great. If you want people to still make use of volks your going to HAVE to give them something that makes people want to make them, otherwise your just going to see Sturmpio spam and more Kubels.
The reason Volk's don't have AI weapon upgrade is because they already have an upgrade in the form of a shrek. Balance is a game of take and give, nerf and buff. Volks suck at AI early game but excel at taking out Vehicles when massed in numbers. The logical conclusion of taking away their purpose is that you have to give them a new one, or else you end up with the SU-76, a unit with no purpose.