And a scenario we can easily imagine is that a buggy update goes out to the whole fleet overnight that starts killing people all over the place.

The common case of accidents being on par with manual human driving goes out the window until the software is rolled back and for 12 hours, 24 hours, however long, we get a number of deaths that far outpaces what humans are capable of. The "worst case" would never apply to a manual/human population as a whole, at once.