Yes, in that scenario, the right number of hours to maximize productivity and balance with the other things in life is going to be calculated differently.
But for most people, we are working for an employer. We have nearly no control over what our work will entail. We do not give a single flying rat's behind about that work. We do that work solely because capitalism requires us to do some work, and so we do for survival. In this scenario, the correct number of hours to spend working is as few hours we we can get away with while still drawing a salary.