It's a decent last-mile solution rather than a replacement for things like trains. Think about the Livermore BART extension I mentioned that will never be built. That route is currently served by a 40m bus that runs twice an hour and sees approx. 860 riders / day (latest numbers I could find, covid ridership) across its 27 stops. Most of those buses are functionally empty.

An autonomous vehicle could do the same route in 15m (so people might actually use it) and it wouldn't be empty.