1) This isn't buffing pathfinders to be a replacement, it's making them useful without being a replacement.
2) You already got a nerf to rifles and the Stuart. Nerf them any more and USF will fall apart.
Again Pathfinder are inline with JLIRS and are balanced, they would be used allot more if they where available in any other army. There is no real need to buff them.
The Stuart has nothing to do with riflemen performance or infantry balance. The nerf to riflemen was at vet 3 so it has little impact on early infantry fights.
If and when riflemen are toned down the road will be open for other units to be buffed if there is a need to be buffed. Unit balance and faction balance are separate issues most of the time unless directly implement in faction design.
Compared to USF most Ostheer and Soviet infantry seem like a to UP.