Volks can't outlast Cons late game.
They are not supposed to.
7 man cons was made
specifically to outlast other infantry, focus fire and explosives is how you counter them, not small arms, cover to cover small arms duels is how you lose to cons in late game.
That's the focal point of my comparison, that there is a disparity between the cost of Volks and the performance of Volks (or, if you'd like the other way around, the cost of 7-man Cons and the performance of 7-man Cons, but I would consider it more the former).
Volks vastly outperform cons across the whole game until cons get 7th man. You have value disparity that goes together with performance disparity in favor of volks for the whole game where volks are meant to operate alone.
There would be balance issue if 7 man volks arrived at the same time as stg volks, but that clearly is not the case.