If right now a lot of peoples disengagements are like mine-- mostly comfort/routing problems rather than safety ones-- and V12 fixes THOSE (but not the safety ones) then you'll see a large drop in disengagement while not actually getting ANY improvement in safety for example.
Whereas if most disengagements today ARE for safety, and you see a huge drop, then there is such an improvement.
But you have no way to know if FSDb disengagements are "safety critical" or not.
Thus a change in rate does not, inherently, tell you much useful about potential accident rates.
Especially when there's entire pieces of the system that do not exist yet to be able to be an RT (complete OEDR and an L4 fallback capability to name two). Right now the human is doing those tasks. We've no idea how well the CAR can do them until the car starts doing them.