And, for a car driven any reasonable amount, most of the cost is in the mileage.
Neither, any savings will trickle up to the investors. The price of robotaxis is going to be just below the limit where it would make sense to own a car.
I think most people will try the taxi, but if you already own a car (that is transit doesn't make sense for most trips) you will discover it isn't much cheaper than owning your own car, and your own car is waiting outside when you want to go (one big advantage of owning a car over transit is the car is ready when you want to go instead of having to call or hope one is waiting - if cars need to wait outside your office all day in case your kid gets sick that increases costs). Instead you can just buy a self driving car and then leave your things in the car if you go shopping over lunch - something you cannot do with a shared car.
Cost. The cost of an Uber is way too much for daily travel (vs owning your own car or public transport).
A human-driven taxi needs to pay the driver's salary within an 8 hour shift. An autonomous taxi can run (almost) 24/7, 365 days a year. Which do you think will be the cheaper fare?
Another scenario is someone simply renting out their own car as an autonomous taxi whenever they aren't using it themselves (which is most of the time). Then it'll always be cheaper than current-day taxis because it's just a low-effort bonus source of income to the car owner.