A bus could serve 40-100 daily. The infrastructure already exists - the same as for cars. It's pretty low cost also - especially electric buses that have lower maintenance costs. The problem is that you have to ride with other people.
Imo, the biggest problem is that of convenience. Convenience (time), cost, comfort, and (perceived) safety are the factors people use to make transportation mode choices.
If buses aren't prioritized in traffic above single occupant vehicles via dedicated bus lanes, signalling, etc. then they'll always be slower than a car because of the additional stops. Frequency is also a consideration that factors into timing, as well as transfers.
If any US city wants to get people onto transit, they need to prioritize transit above less efficient modes, amongst other things.
Trying to keep using something that is comprised by fixed routes, schedules, and stops makes n little sense.
We don't need to optimise for space, we need to optimise for cost of transport, and door to door duration.
At a slightly smaller scale, think about rush hour. The passengers on a single bus could fill up the entire block if they were in individual cars instead.
And that's not even considering empty AVs driving around looking for a fare. Locally we're thinking about assessing a mileage fee to discourage this behavior, which is a good start, but at some point you inevitably will have lots of empty cars heading to the stadium or whatever when the game is about to end, clogging streets.
I'm definitely not anti-Waymo. I want them to be legalized here. I'm just realistic that they will basically serve the role of taxis/Ubers, not replace the concept of buses.