In New York City, a taxi driver takes home maybe 50% of the money collected in fares after costs (whether depreciation on their own car, renting a taxi by the day or week, gas, etc.) That may be high but it's a nice round number.
If we take this as a general rule of thumb (and NYC cabbies make more than the average), that means you could but cab fares by roughly 50% by eliminating the driver. Maybe that forces a behavioral change on the part of some urbanites. But I can pretty much guarantee you that cutting cab prices by 50% will minimally affect behaviors in most places.
And that's not even counting the many places where having the most rudimentary cab services is barely a cost-effective business today.
Sure, you can speculate about optimized taxi designs (which aren't especially relevant to non-core urban areas with high car ownership today) but small electric vehicles could exist without self-driving too.
IMO this is an academic exercise for at least a couple of decades in any case but it's an interesting thought experiment.