Hopefully in a few years I'll almost never need to hire a taxi, because my car will be able to drive itself during those times when I can't drive it.

Why would people pay more for a human driver? If anything I would think robot drivers would cost more. More likely it'll just cost the same, though, if both are even offered. (Maybe you'll have a choice: Robot only, human-driven only, or first available. Might make sense if there are areas served that require human drivers. The human drivers could then serve people who choose "first available" when they're not needed elsewhere. I'd probably usually choose first available, but might choose robot only if I want my privacy or if I have a 3+ passengers and want extra room. Maybe human-driven only if I'm away from home and want to chat with a local, but it's hard to think of a situation where I'd choose that. If demand for it is too low, and it probably will be, human-driven only won't be an option. Maybe robot-only or first-available will be the only two options.)

One thing you don't seem to have mentioned is the possibility to rent a robocar that you can drive yourself. That may be the best of both worlds until we have robocars that can do everything a human can do.

While I say above that I don't expect to use a robotaxi very much, I might rent a robocar from time to time, if I'm out of town without my car, or if the rental vehicle can do a lot of things that my regular car can't (including technologies like self-driving but also things like hauling large objects or seating six comfortably or going 500 miles without charging or camping in the back or on top). Maybe you could call some of them hiring a robotaxi service. Renting a car that seats six for half an hour to take the office to lunch is essentially a taxi service. Even then I'd expect much more flexibility than taxis traditionally give you. (It might take us 15 minutes to get in the van.) The lines between taxis and rental cars blur when the car drives itself.

Rentals will become so much better when pickup and drop off can happen at a convenient time and in a convenient location, because the car can drive itself between the storage area and that convenient location (even if it can't drive itself everywhere you need it to be).

If, as you suggest, depreciation on these vehicles will be primarily by-the-mile, the hourly rate to rent them when you're not driving should be low.

Maybe driverless robocars will quickly move from "available in limited locations" to "available just about everywhere," but if not there will be a strong market for renting a car that can be used both in robocar mode and in ADAS mode.