This is an interesting point, but I'm not sure that I agree. If humans sometimes drive impaired and Waymo never does, then that is a real safety point in favor of Waymo. Try as we might, we haven't been able to eliminate drunk driving by humans. If Waymo vehicles just don't have that failure mode, that's a meaningful improvement.

Consider this: suppose that Waymo drives more safely than "US drivers overall" but slightly less safely than "US drivers who never drive impaired". Then… replacing a uniform slice of US drivers with Waymo vehicles still improves safety! It would only be problematic if you somehow adverse-selected by replacing "the safest" human drivers with Waymo but letting the most dangerous ones continue to drive by themselves. Which is interesting to consider, but I don't see why it would be the case.