If they modify any part of the stolen base logic, they're going to have to modify all of it.
In your suggestion, if bad base runners are prevented from ever attempting to steal, then the overall percentage of successful SBs is going to go up past the point of being realistic compared to MLB.
So then they'll have to modify the existing logic to throw out more of the high speed and high base running guys. Or maybe make the guys just above the cutoff attempt more SBs and thus get thrown out more often. Or just let the SB success rate move away from MLB norms.
If they're going to do that coding, might as well just change it to individual settings. And they've been pretty clear that will never happen.