More than any other change in the car industry, autonomous taxi services would make things massively better. Once you make it cheaper and more convenient for most people to use the taxi service than to own your own car, then:
You can customize your car to the journey. One-person 3km trip? Hail a short-range one-person electric car. No need to drive a big car just because you need it once in a while. This leads to a massive reduction in average vehicle size on the roads, and big reductions in energy consumption.
Most people won't need driveways and garages anymore. Extend your house or put in a garden.
Large reduction in the need for on-street and off-street parking. Replace on-street parking with pickup/dropoff bays and/or bike lanes and/or more general lanes.
On one hand, this would reduce public transport usage by providing a more convenient alternative for many people. On the other hand, this would increase public transport usage by reducing instances where people drive from A to B only because they'll need their car to get from B to C. Both of those cases are wins.