The improvement lays in the flexibility of the system. In the financial district, you have a large station with similar costs and restraints as existing systems, and with multiple paralel lanes to handle the trafic, merging etc.
When you fan out throughout the city, you can build, for the same price of a single subway tunnel, multiple links, and you can build much more micostations, close to where people need to go, with very short ramps and deceleration and acceleration handled cooperatively with the other vehicles, since throughput is no longer critical.
And finally, on the last mile, the vehicles goes outside and drives to your suburban home on the existing road network, in areas of low density where no people mover will ever make economic sense.
It is this flexibility that makes self driving vehicles a very practical proposition for future cities.
