car autopilots might reduce emergencies and fatalities while making humans worse at dealing with emergencies in general

In April, at an event in Norway, Musk said, "The probability of having an accident is 50% lower if you have Autopilot on," according to Electrek.
That may be, but data isn't the only consideration. When human lives are at stake, perception and emotion come into play. Automated driving systems will have to be demonstrably better than human drivers before people trust them with their lives.
Yet, perfection is too much to expect from autopilot systems. Machines fail, and failible people are likely to remain in the loop. In aviation, automation is common and has prompted concerns that it degrades the skills pilots need when intervention is called for. If the same holds true for cars with autopilot systems, we can expect to become worse drivers, less able to respond to emergencies, even as our autopilot systems reduce fatalities overall.
There may be no getting around the fact that, given current vehicle designs, driving down a highway at high speed entails some degree of risk, whether a person or a computer is at the wheel.

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Tesla Autopilot Crash Under NHTSA Investigation

Having a car autopilot partly responsible for driving while a human is also partly responsible for driving could be a deadly combination. 

It is better if a human is totally responsible for everything, that way when something happens it is not ambiguous when a human needs to step in and save lives when an autopilot doesn't detect an actual problem. Also, how quickly can the human step in and save lives? Even with cruise control on a car right now there might be a moment's lag time which might be too much for a 70 mph situation for the human driver to rescue the car and it's occupants from danger. However, most car's cruise controls immediately disengage when brakes are applied.