Still not addressing the point where the government doesn't have the authority to mandate this.
Most cars will still have the option, but as insurance companies see that those cars with autonomous ability get in fewer accidents, insurance rates will drop accordingly. Eventually, in the distant long term, it will be the autonomous ability that comes standard on all cars, and the manual ability that is an optional feature.
I don't think having a manual driving option should be illegal and I don't think it would be made illegal. You might have some legal restrictions on manual driving though in the form of automated-driving-only lanes, similar to HOV lanes today. An automated driving lane could go 120 mph and have less space between cars (in good weather, of course).
Regarding the quality of GPS units, if you watched the video I posted earlier then it is clear that there is a huge difference between a GPS unit that is essentially blindly reading a map and the automated car that can see everything around it and adjust accordingly.
It is also silly to think that kinks won't be worked out such as the software knowing which dirt roads will be able to support the vehicle and which will not. It really surprises me that anyone who has been awake during the past few decades does not understand that software improves.