What I've found is that I spend the 4 days I'm not "working" organizing my thoughts and plans as a background task and this means most of my 3 days is spent executing. Whereas at my previous job I would spend several days a week on thinking and organizing and another couple on being blocked or bored.
Have you, and how did it go over, if so?
If that's so easy, why not stop doing those things and keep the 5-day week?
I don't know whether that's true, but it's a plausible argument for why people would do less of the unproductive things you mentioned.
In other words: If people at work 40 hours a week on average, but only have the mental stamina for 32 hours of work a week on average, why stay in the office 40 hours on average? Those 8 extra hours will not account for extra productivity. Switching to 5 day weeks, in this model, won't increase productivity, nor 6 day weeks.
I've said "on average" a lot, and that's very deliberate. Most people I've ever worked with across 20 years in tech are largely unable to keep up a fully-mentally-engaged 40 hour work week every week, and even fewer are able to sustain longer average weeks than that for very long at all.