I don't understand how paying attention can prevent this sort of thing.

Let's say you're paying attention with all your might. You see something up ahead, like an overturned tractor trailer. You expect autopilot will sense it, but you're preparing for it not to. But how do you know it's not going to sense it? Well, obviously, when it hasn't started reacting in a reasonable time. But then it's too late!

In order for a human to have time to figure out that the software isn't working it would have to routinely operate with (much) slower reflexes than a human. But then it would be useless, as you can't slow down normal driving, and in any case, that's not how they designed it.

The hypothesis that people have unreasonable expectations of something called "Autopilot" seems unnecessary and irrelevant to me.