The claim to be answered is not 'people can't be productive working more than 40 hours a week'. It's that people would be more productive if they worked less. We don't have to rely on vague anecdotes and 'what about people who are really really sure and feel it very deeply that their personal sacrifice wasn't counter-productive? Shouldn't we just believe them so they can feel their exertion wasn't wasted?' We've got research.
But its quite important to make a distinction between manufacturing work, repetitive physical labor, and mental work. Different organs are involved and they function differently. As nice as some might imagine it would be to only employ machines, in reality we still have to put bathrooms in the workplace, give people a lunch break, and admit that they're sacks of meat. They've got limits, and ignoring them will never be productive.
But its quite important to make a distinction between manufacturing work, repetitive physical labor, and mental work. Different organs are involved and they function differently. As nice as some might imagine it would be to only employ machines, in reality we still have to put bathrooms in the workplace, give people a lunch break, and admit that they're sacks of meat. They've got limits, and ignoring them will never be productive.