Just because you might be less productive after 8 hours does not mean that you cease producing measurable results.
You WILL build more if you are building every waking hour. If you are obsessed enough to do so.
You could work 14 hours a day and do the work that someone else could do in 4 hours. But that is not what this discussion is really about. This discussion is about the fact that if you are crazy, hungry, obsessed, and single minded enough to work 100 hours a week, intelligently, you WILL accomplish far, far more than what anyone else could ever do in a 40 hour work week.
You're making some assumptions that are shown to be incorrect.
1) You're assuming that you can start every workday at maximum productivity regardless of your previous workload. This has been shown to be false. Previous overwork will make you start your day already at depressed productivity.
2) You're asuming that there is no way that somebody who works 2.5x as many hours can acomplish less than in 1x as many hours. This can also shown to be wrong in general because your productivity due to overwork can drop below 40% of what you can achieve when rested. It can also shown to be especially wrong for work where the quality of the work has a future compounding effect (such as programming). If you make bad decisions one day, because you are overworked, even if you achieve more that day than a rested person would, the bad decisions can cost you all the gains and more the next day, weeks or months. I.e. not only will you have achieved less, you factually worked backwards.
You WILL build more if you are building every waking hour. If you are obsessed enough to do so.
You could work 14 hours a day and do the work that someone else could do in 4 hours. But that is not what this discussion is really about. This discussion is about the fact that if you are crazy, hungry, obsessed, and single minded enough to work 100 hours a week, intelligently, you WILL accomplish far, far more than what anyone else could ever do in a 40 hour work week.