"Super simple" would be nice, but as I understand it, even "dumb car" software is incredibly huge and complicated. (It's very unclear to me why that needs to be so)
If self-driving cars really take of, "just be a dumb car" won't help in a few decades. The 'driver' may not know how to drive his car, and even if he did, the car may not have a steering wheel.
Also, I guess one would press this 'I want to take control' button in an emergency. That means the persons suddenly finding themselves driving this vehicle have to be above-average drivers.