What Waymo et al have now probably is sufficient for a "public transport" role, given the fixed service area and high density enabling investment in mapping infrastructure with manual fixes and local heuristics.

Erm. Waymo exists already. It's not perfect, but it works. You can get a ride in an actual human-free taxi in SF.

Extending Waymo to handle ALL possible situations autonomously (you put in "New York" in the address field and the car drives you there from Seattle) is probably a too big ask, but working within a pre-mapped area on semi-fixed routes seems plausible.