I disagree with you here. Elite anti-infantry specialists only buildable from 12 to 15 mins onwards being able to counter mainline 260mp infantry shouldn't be seen as a problem.
Basically what you want is Riflemen to be way too good - infantry that scales perfectly and has no weaknesses at any point in the game. Their current late game stats would be completely unacceptable for a 260mp unit.
Riflemen are designed right now to start off very weak, become decent, and become super powerful late game. Volks are good early game and have an STG power-spike in midgame, but are mediocre late game. The asymmetry in when they get their power boosts is problematic for the USF vs OKW matchup, but making Riflemen overly cost-effective is only going to swing things the other way with USF becoming too overpowering in infantry engagements.
lol. So your solution is to keep Riflemen early game being somewhat equal to what they are today but cheaper, so they don't have the upper hand on volks. And then late game, when they would start to get that upper hand being ripe off by falls/obers because reason.
But since you say it shouldn't be a problem, just explain yourself: how.
Riflemen have never being design to be weak early game, quite the opposite in fact. They always been so strong that the entire USF design revolved around paywalls and hard strategical decision to mitigate their potential. The situation we are today is the result of late design to make all factions equal early-to-late game but never took in consideration that USF is laking tools and struggling with a specific design that can't work anymore the moment your remove the advantage to riflemen.