> Or perhaps employees were output-limited not by mental fatigue but by, essentially, giving-a-shit. One of the big insights of productivity studies is that "can't do more" and "won't do more" aren't clearly distinct when you're relying on internal motivation; there's no clear way to say that "won't" is happening on a different, volitional layer. If people were working 40 hours a week with 25 hours of 'drive', constant productivity makes more sense.

I think this makes the most sense. Assuming I understand right what you mean, it could be described more simply as "work expands to fill the available time", a common enough criticism of sprints in agile (cases lasting longer than they normally would because there's still days left in the spring).