That's an oversimplification of the reality. The human body doesn't come with a dipstick to check if your brain has been topped up with enough rest to get you to your destination.
I've started plenty of duty periods where I felt adequately rested to complete a maximum duty day, only to find later on that I was fighting to stay awake during the latter portions of the flight. If those duty days had been shorter, there's less chance I'd have felt that way.