Recently I was flying with a guy who when the auto thrust kicked off and would not reengage in cruise, wanted to hold and start a dialogue with maintenance about a fix. We were only a few miles from top of descent at our own hub, and he didn't feel comfortable taking it down by hand. We did some resets to no avail and he looked at me and asked what I thought we should do. I guess the years in an RJ paid off because at least one of us has flown a jet from cruise by magically moving those two sticks in the middle to keep the fans blowing.
Funny story. What did your FO propose as the solution to the problem? Declare an emergency?
Just to be clear, I don't use the term "task saturated" as an excuse for lazy, but on the last flight of a long duty day, I'm more likely to use the AP to the fullest extent.