That said, it's essentially impossible to get rid of cars entirely, and you can see that just by looking at countries or even regions that have good transit and notice that they still have tons of cars. Even Singapore has lots of cars, and the entire country is a super-dense city-state!
So it's definitely worthwhile to develop self-driving cars and trucks, even if you have fantastic transit and biking and walking available.
Bus drivers are a limited and very moveable workforce, and if you have huge economic differences in your common market, poorer cities won't find enough drivers to sustain their own systems.
An example: there is helluva lot Bulgarian and Romanian drivers in Western Europe, and there is no way that Sofia or Bucharest, cities with half the GDP per capita, can afford to pay them competitive salaries.