Earlier quoted context omitted.
I'd heard that the actual reliable sensors that can see through rain and such are like $50k+ so are not affordable in private cars, but a service that uses it all the time could. I wouldn't want to fall asleep with today's consumer-grade hardware, but you're right that even an autopilot that has to be supervised still has most of the safety benefits.
You could also let the cars network, to leverage multiple sensors (some of them way ahead of your car).
How would you verify that the other cars are sending you reliable information?