But it's worth noting how leisure hours have been allocated after the invention of the 5 hour work week: we've reduced working hours at the end of life (longer retirements), start of life (longer education), and some amount of people simply do not work.

There hasn't been a reduction in hours during peak earning potential because many jobs are competitive, because firms are in competition with each other.

Maybe some companies will start doing 4 day work weeks because they find that productivity doesn't actually increase from 4 to 5 days and then start outcompeting other companies for talent. But unless 5 days is actually not more productive than 4 days, we're going to have the most competitive organizations continue to be 5 days a week.