Waymo made a very good point about this, fundamentally that's the problem with a progressive rollout - drivers will stop paying attention when the software is "

Waymo made a very good point about this, fundamentally that's the problem with a progressive rollout - drivers will stop paying attention when the software is "good enough", and the result will probably be crashes. So for them, it was all or nothing - either it fully drives itself, or it's not worth deploying.