Yeah if you do the math you end up realizing you need to run the cars like airlines run planes - damn near continuously. Idle time is non paid time, and you need to keep everything moving.
The real advantage of self driving taxis isn’t going to be the self driving, it’s going to be the 24 hour/day shifts they can pull.
That doesn't really work for taxis. No matter how much you want to distribute rides across the whole day, no discount is going to be able to get people to commute to work at midnight - most of your rides will happen around 9am and 5pm. So many taxis will have to spend a good chunk of the day idle. With self-driving taxis, at least you're not paying human drivers for some of this idle time (which Uber doesn't either AFAIK, but traditional taxis do).
This is in contrast to air travel, where with the right discounts you can get people to fly at inconvenient times of day.
I think most of rides by volume will be around weekend and special holidays. But even then limited 2h-4h slots, around bar opening and closing times. Having enough supply for these peaks will be impossible to make work.
The real advantage of self driving taxis isn’t going to be the self driving, it’s going to be the 24 hour/day shifts they can pull.