I think that line is going to a bit more blurry than you think. Waymo has already done fully driverless testing during the early rider program, with plans to roll that out to Waymo One in the future[1]:
> We’re never done learning, and our early rider program will continue as a way for a select group to give us ongoing insights. They’ll help test early features before those new capabilities graduate to Waymo One. For example, we’ve already offered fully driverless rides to some of our early riders.
And the number of people with access to Waymo One is going to steadily increase over time[1]:
> We’ll first offer Waymo One to hundreds of early riders who have already been using our technology. Over time, we hope to make Waymo One available to even more members of the public as we add vehicles and drive in more places.
So with that in mind, let's imagine if, for example, at some point in the near future 10,000 people have access to Waymo One and 1% of Waymo One rides are being completed without a safety driver. Would that satisfy your milestone? What about 100,000 people with 10% driverless rides? If Waymo eventually reaches a point where 95% of rides are fully driverless, and 1 million people have access to the app, would Waymo opening the app up to everyone _really_ seem like that significant of a milestone anymore?
[1]: https://medium.com/waymo/waymo-one-the-next-step-on-our-self...