If fewer people die, there is only less liability to assign. Remember that the original argument was "FSD needs to have 0 fatalities before we adopt it". And it's not valid to argue that some traffic fatalities are unavoidable; I know people who work at the DOT and "Vision Zero"[0] is a thing, and already implemented in some European countries/cities. Without FSD. So FSD needs to clear this bar.

[0] https://visionzeronetwork.org/about/what-is-vision-zero/