My experience with autopilot isn't that I lose focus as much as I focus on something else. Instead of focusing on keeping in my lane, not going too fast, and if the car ahead of me is braking, I'm focusing on the road ahead -- is there construction coming up? Is there debris in the road? Is there a wall ahead that gets too close to the road? Is there anything weird up ahead that needs attention? You learn what AP can and can't handle and you keep an eye out for anything out of the ordinary, which on long road trips is significantly less fatiguing and probably safer.