The elephant in the room is that if moving to a 4-day week from a 5-day week is neutral then it shows the amount of slack people have in their 5-day week...
More accurate conclusion is that the upper limit for efficient working hours is far less than the standard 40. This underlines how ridiculous the people who claim to work for even more than that already too high amount.
That's your opinion, not a "more accurate" conclusion.
I think the upper limit, really is consecutive hours before you need a good rest to be able to start over. If you can do that over 4 days, you can do it over 5. A standard 7-8 hours a day allows both for breaks during the day, and good rest overnight so I don't believe that it is "ridiculous" to think that it is sustainable over 5 days. (Happy to be proved wrong).
As an anecdote or personal experience, I think we've all been in offices (all/most offices?) where people were not exactly flat out all day long...
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I think the upper limit, really is consecutive hours before you need a good rest to be able to start over. If you can do that over 4 days, you can do it over 5. A standard 7-8 hours a day allows both for breaks during the day, and good rest overnight so I don't believe that it is "ridiculous" to think that it is sustainable over 5 days. (Happy to be proved wrong).
As an anecdote or personal experience, I think we've all been in offices (all/most offices?) where people were not exactly flat out all day long...