Recent progress toward shorter hours among certain sectors of UK workers has set a starting point for a winnable demand for workers everywhere: a four-day workweek.
Worked well for me. Altho I'm just a small example. I run a mechanic shop where pay is directly tied to productivity, not time. About 6 months ago as an experiment, I started giving the techs off on fridays, making it mon-thurs. Turns out overall productivity didn't change; they got more done per day on the 4 day work week. So it ended up being an extra day off with no change in pay. They're happy with the longer weekend, and I'm happy with a day of peace and quiet to get paperwork done. I don't plan on changing it back anytime soon.
You point out the “problem”. “Productivity” and “hours at work” are decoupled in your situation, but as a culture we generally don’t believe that to be true. We exalt the people that “burn the midnight oil” and “stay late to get things done” because we assume more hours equals more work getting done. Until we break that culture, a 4 day work week is not going to be widely accepted.