I've heard that Sweden did a research about 6 hours long work day (not the same thing as less work days I know).
The results were simply that the workers were more happy and more efficient.
They get it, but to them the only good worker is one who is well controlled.
If a work week of thirty two hours would be proved equally productive as one of forty, if most in society would be caused no harm from such a reduction, then workers may begin shortly after to consider a twenty hour work week.
Then, while considered the new objective, workers also may be discovering new opportunities for self care and community care, developing new relationships with hobbies and leisure, and expanding their identities into new facets and in new directions.
After not too much time would pass, a critical mass of workers might start to feel convinced that the whole system is a house of cards, built only on threat and deception, and deserving be dismantled in favor of one that is new and different.
Personally I would much prefer to have a 6h work day or 6,5 hour (for it to be 32h, like the 4-day work week) than to have 8 hours a day for 4 days. I don't care about having one more day of free time if I still don't have as much time during 4 days of the week. I would much rather work less time those 5 days so that I actually have time to cook, exercise and do my shit every single day.
I work 7h a day, some day soon I will go to 6.5. I would always choose that over a 4 day work week. Because now I have actual evenings where I can do things. 7 days a week.