It is just illustrating that the main reason more people don't have amphibious cars is not so much the expense but lack of desire to have one rather than prohibitive cost.
I could go into the system engineering of a complete drive by wire automobile, along with the actuators, redundant position sensors, and required onboard computers to run thing, but you have solved the problem by adding some manual controls.
I do not see the problem. Some cars (obviously not self driving) already have drive-by-wire technology. Having a system where electronic commands to steering, braking etc. actuators come from human operated controls or a computer vs. one where they only come from a computer is not very difficult or expensive. I can definitely see the steering wheel and a set of pedals being sold as a $1000-$2000 option on a new self-driving car, depending on how fancy the actual car is.
It's not a matter of whether it can be done or not. It's a matter of will it be worth the expense to create a system which accommodates self driven cars and human driven cars on the same road. It will be very expensive.
You are mixing two things here. Additional cost of the car where you can choose manual or automatic driving vs. the whole system populated by a mixture of self-driving and manual cars vs. roads only being populated by self-driving cars.
As I said before, the first is not as big an expense as you think. As far as the second, I can see some benefits having only self-driving cars on the roads. However, that would only be possible to even think about if self-driving cars have sufficient market penetration. First self-driving cars will by necessity share the roads with mostly regular cars (whether or not these self-driving cars have manual controls or not) and it will take decades after that to achieve sufficient market penetration to even contemplate banning manual driving.
