Even if the answer there is "never", so what? Do we really need to set the bar at "replace all vehicles everywhere in all situations" for it to be a success?
It becomes much harder to justify buying a self-driving car if there are many parts of the country where I can't drive it. Even a mid-tier sedan can be driven all the way up the west coast from southern california to Seattle as long as you have chains and wait for the pass to be open. Waymo's cars are designed to not even have steering wheels, last time I checked...
Most likely due to Waymo being not designed for personal consumer use... but I’d also say they are pretty detached from having to deal with real societal problems.
Self driving cars is indeed a cool technical problem to solve if you asked an engineer, but if you asked society what it needed I doubt self driving cars would be the answer.