I believe most people are inconsistent drivers. Usually they do a reasonable job, but some fraction of the time they're worried about a sick kid, or a job interview, or a recent breakup, or whatever, and they aren't paying attention.
There are some truly consistently horrible drivers. But in general most people do ok most of the time. I think the real challenge is cutting of that really bad end of the distribution, which autopilot seems to be doing.
Those same drivers are, unfortunately, also likely to be those least amenable to actually using it. Some people I know have expressed disgust toward the idea of autonomous driving; coincidentally, almost all are horrible drivers that I wouldn't want to be with as a passenger.
There are some truly consistently horrible drivers. But in general most people do ok most of the time. I think the real challenge is cutting of that really bad end of the distribution, which autopilot seems to be doing.