I can tell you that I have worked definitely at my 'resource' limit a lot of times the last months and it definitely wasn't due to insufficient motivation but to plain exhaustion.

Actually when the motivation for the main task subsided I'd temporarily switch to a more rewarding intermediate task like lesswrong, thus the opportunity model itsn't wrong either.

But there comes a time when Isshoukenmei takes its toll. When you have to realize that your concentration is slipping after 14 hours and all further effort nearly ruins previous work.

But I have to agree that this amount of exhaustion is seldom nowadays and in your example of the 'exhausting' 8 hour work-day the 'exhaustion' most likely is from dissatisfaction, lack of reward or misapplication of pressure.