I think you overestimate how willing someone is to sit in a small car with 3 other strangers during their morning and evening commutes. I don't see a huge impact on ownership at least in the first decade or two of autonomous cars. Lots of people still have relatively specific needs, e.g. requiring car seats of various sizes for their young children, needing to pick-up or drop-off kids at school, sports practice, etc.
Mass transit works well in dense areas because the vehicles are large and only need to go short distances or go long distance over a specific route. When everyone has their own custom route, person A's route may make person B's route unacceptably longer. Also, sitting on a bus/train with 50 strangers is less of an odd social situation than sitting in a car with one stranger.
Once autonomous cars get to the point where they don't require any driver attention at all, I think it will make ownership even more appealing since you could customize the entire to match whatever you want to do while you're hauled around.
Mass transit works well in dense areas because the vehicles are large and only need to go short distances or go long distance over a specific route. When everyone has their own custom route, person A's route may make person B's route unacceptably longer. Also, sitting on a bus/train with 50 strangers is less of an odd social situation than sitting in a car with one stranger.
Once autonomous cars get to the point where they don't require any driver attention at all, I think it will make ownership even more appealing since you could customize the entire to match whatever you want to do while you're hauled around.