Studies have shown time and time again that a 4 day work week results in higher productivity and happier employees. Happy employees also do better work.
It's a non-sensical argument. If a 4 day work week is more productive than a 5 day work week it means the 5th days is producing negative work. You could literally do nothing every Friday and you would not be less productive than a 4 day work week.
Or it could mean that increasing weekly time off by 50% improves people's happiness and productivity, or that a 5 day work week results in days of work that are less productive than in a 4 day work week.
This would be an accurate statement for most work places. People are often creating negative value in the modern workplace in the form on tech debt, new bugs, unwanted features, meetings that drain the rest of the staff...
5 days isn't productive for knowledge and creative work, it leaves people in a state of perpetual burn out and greatly lowers the quality of output.
Your take is equally non-sensical. There is no magical constant unit of time that represents maximum productivity. It’s all relative and fluid. It’s based on many many variables. And even if such a time existed for one person in one company for one week, it wouldn’t be neatly packaged like “40 hours”. It would likely seem random like 34 hours 20 minutes and 25 seconds.
Arguing over such a universal time is as much a waste a time as arguing over the perfect diet. It’s relative.
It also sounds like you're not familiar with simple ideas like Parkinson’s Law. Listen to yourself, you don't think people are fucking off at work? It's business hours and you're on hacker news not doing work, case and point.