> The additional hardware and software will be expensive as hell to develop, but once it's invented, the marginal cost should be fairly cheap compared to the price of a car.

Go talk to a pilot and ask them about Garmin products and the autopilot of their plane. Also ask them about update the software on these devices. Ask how much money they are asked to fork over for software...

Have a look at the self-driving tech of the John Deere tractors and how well that has gone for the farmers. I don't think the bulk of people here have really thought about this in a negative light.