I'd be curious to see any numbers on that. Uber and Lyft played the card of "but it's car sharing and thus reducing trafic" but in the end, it doesn't really change much, and sometimes even increase trafic.
https://www.cnet.com/roadshow/news/uber-lyft-traffic-congestion-car-ownership-study/
As someone that doesn't have a licence, never have driven a car and avoids them if possible, taxis (autonomous or not) are generally useless to me. They are still cars and they keep people dependent on cars.
In a city, people shouldn't need a taxi (read, a car) to get around. In urban areas, people can walk, bike or use public transit. And if you want to go from one city to another, one shouldn't also need a taxi (car) to get there.
I'm aware that this is kind of "utopist" and requires efforts to change things but, if the solution to wanting less cars is "cars but shared!", this is not a solution. It's just proof of failed policies. It will not encourage any change, just stagnation. Things will stay based on cars.... but autonomous... and electric (oh so green)! Oh and also, now big tech knows exactly where you're going, and when.
EDIT: And if robotaxis would be "part of the solution" for the last mile, why wouldn't a normal taxi already fulfil that role? We don't have enough? So we'd need more vehicles on the road, But autonomous? Why do they need to be autonomous? How come is the last mile such a huge problem for most people? Is walking or cycling a mile that difficult for most people? Is everyone carrying a 42" TV around?