People use public transport in villages of 20 people where I live. The idea that you need some mega city for public transport to be viable is an American fantasy. Even if its not for everybody, it still makes sense.

Also most people live plenty urban to make public transit perfectly viable.

Self driving cars means even more cars on the road, and reduce the avg occupation even more. Even in a same rural environment this isn't great. And in an urban environment is insanely fucking stupid.