Indeed cruising for a little bit while the driver does a quick errant might be better than parking when it's pricey. We do that all times when there is more than a driver in the car.
However, it feels arrogant to me to decide that when self-driving is a thing the cars will just clog the roads cruising while the drivers aren't there.
This ignores a bunch of things. Here is an unexhaustive list: a. The risk of collisions is greater driving than parked. b. Autonomous ride-sharing services would probably be a thing in this scenario (be it running some errands while the owner is busy or just deployed full-time like this). c. There might cheaper denser far away garages that the vehicle could just immediately head to thanks to availability awareness. d. It is very unlikely that the auto sector would program vehicles to clog traffic like this on purpose (though someone hacking their car to keep running nearby might happen). e. Parking in San Francisco is very expensive. However, San Francisco is not the only city in the world and cities are vasty different from one another. Usually, for example, they are not islands and vehicles could drive, say, 10min to a parking lot on the nearest city (maybe giving a ride to someone else in the way).
I'm not saying that we should not consider doom scenarios, but reality has shown it most central planners fear too many things that don't happen often. We all use the Internet today, and it is relatively easy to fire a few threatening messages to people around the power to terrorize them (and get away with it). However, it just barely happens if you count in statistics.