The person who assembles 80 bikes in 80 hours is exceptional. Rare.
You can't count on having a workforce of such people.
And that person then gets promoted to supervisor, to higher and train other assemblers.
But he's not likely to find an 80 bike a week employee.
I will agree that in general if you work more hours you will get more done.
But there are diminishing returns where you get to a point where an extra hour yields no more productivity because your pace and interest has slowed.
And your hourly wage just goes down as you work longer on salary.
Pay the workers per bike instead of per hour.
Then you will really find out what he optimal work week is for each person.