Self-driving cars that only go 80% of the way would compete against those forms of transportation which have established business models, networks and in the case of trains are subsidized in a way that self-driving cars likely won't ever be.

Additionally the key advantage of cars is flexibility, not having to figure out how to get to or from the train station or airport. In your scenario self-driving cars would lack this advantage.

Now granted this may still sound like an appealing proposition in countries with poor public transportation infrastructure but that substantially reduces the size of the overall market. Sure this might be attractive for the US but what about Europe? Asia?

Now of course there is an obvious answer to this problem: Keep the steering wheel and drive the rest of the way yourself. That just means you'll have a lot of drivers who won't gain experience at the current rate though. Not sure I would like to share the road with those people.