Can they just report directly how many human lives they have saved since beginning operation? (of course, within some error bars)

Maybe that's too much of a statistical stretch.

But would be a good to-the-point number to have on hand for some waymo debates.

"yes they caused some disruption in an intersection in so-and-so scenario, but on the other hand they saved X number of human lives last year"

I think the average is ballpark 1 per 100 million miles, so my guess is that someday they might show this, but they probably want to wait til they have more miles driven.