Why is that not scalable? Mapping out two large cities for an experimental project in a few years seems scalable, expand to new cities over time with additional resources.

I suppose it is, but not in a Silicon Valley way. They could scale to "large Southwestern city taxi service," but it wouldn't earn back the investment or deliver on the hype. If that becomes the ceiling I bet Google will simply shut Waymo down.

If they work out how to deal with, say, New York weather conditions, there's potential, but they don't seem to be any closer.

I think you’re conflating doable and scalable.

Or perhaps my threshhold for “scalable” takes different parameters and weigh these inputs differentfly from you.