A control model shows that such environmental disturbances cannot be a major contributor to error when error rates are low because, the fact that error rates are low means that the control process is already effectively compensating for these disturbances.

Sorry, but that doesn't sound like an interesting result that vindicates PCT. You can even rephrase the general insight without controls terminology!

Like this: "given a system that is demonstrably robust against failure mode X, it's unlikely to fail in mode X".

Positing a "control system" is just unnecessary length and unnecessary delimitation of the general rule. PCT doesn't get you this insight any faster. And while human factors engineers would discourage similarly named, very different drugs, even they would admit it might not be worth fixing if the system has already operated without ever swapping out the drugs.