> with the majority of people who want (or have to) use cars
Car owners are actually the minority in many large cities, especially in the dense parts. (>75% of all households in Manhattan don’t own a car according to https://blog.tstc.org/2017/04/21/car-free-new-york-city/)
Sure people living in the center of a city do not have many reasons to own cars, but they still often use taxi, and some want to have cars but can't afford to. And then some people do not use cars normally, but have to use them when need to transport something heavy, travel with children, or simply due to illness. So assuming everyone who doesn't own car wants them banned is too optimistic.
Disclaimer: i live in a city not in US, i do not own a car (although can afford it), and use uber occasionally.
Tunnels still need large parking lots, which deteriorates the quality of the city for everybody.
The self driving cars don't need as many parking lots, and with cheap tunnelling tech parking lots can be built underground where they want be a problem.
This amount depends on the level of inconvenience of other methods of transportation. Building huge freeways does not simply add infrastructure for cars, but deteriorates the quality all other methods of transportation, and adds huge parking lots. Building underground tunnels with self-driving cars does not create these issues, so should not be judged the same way.
It's easy to say "ban all the cars", but in reality we need to find a compromise with the majority of people who want (or have to) use cars. And for that it is important to remember why we want to reduce the amount of cars, not because we hate cars, but because we want more people to be able to reach their destination more efficiently. We already have some tools for that, and some cities (like Amsterdam) use them efficiently without banning the cars outright. Tunnel with self driving cars is one more tool to improve lives of the people, and rejecting it simply because we do not like cars does not help anyone.