Self-driving people talk about this because, even if the tech worked, the costs of the hardware involved is currently so prohibitively high (~$100,000s per car) that a taxi model is the only one that makes sense.
In which case the capital costs of the vehicle will wipe out a good chunk of the cost savings from removing a human. Therefore, there will be no mass shift away from car ownership.
A $300k robotaxi could still undercut conventional taxi/rideshare, but the costs will have to come down considerably before they challenge the economics of personal vehicle ownership.
The comment beside you says that a car has a useful lifespan of 200,000 miles, or 50ยข per mile for the tech. The internet suggests the average highway speed in the US is 70mph. That suggests that it will cost $35/hr. for the autonomy and probably that much again for the car itself.
At $70/hr., I expect people will still opt to own their own vehicle.
Certainly not, and in many cases it may even cost more than that to drive your own car. But the car comes with a huge advantage that taxis have fallen short on:
The cost is upfront. People sit down and budget $x for transportation and then they know that amount of money will get them anywhere they want to go. People hate having to decide in the moment whether they want to spend the money or not. I notice a similar phenomena when it comes to mobile phone service. People would rather pay extra for a plan that provides more than they could ever need than to have a lesser plan and pay overages when needed because the overages require thinking about it in the moment, rather than pre-planning how much they want to budget. $70/hr. isn't even going to have anyone thinking twice about changing their behaviour completely.
Maybe robs-taxi services will pioneer a pay ahead of time service that captures the necessary mindshare that will make ownership obsolete, but that won't have anything to do with the technology. There is nothing stopping a taxi service from doing that today with human drivers. It's just a tough business model to work with when providing services, so it is uncommon.