> They'd probably feel even better getting paid for five days and working three - and why stop there? Why not two, or one, or zero?

As someone who went from working a 5-day week to a 4-day week, I feel like a 4-day week is a 'sweet spot' - I'm just as productive as I was working a 5-day week, but when I work any less than 30 hours a week (e.g. when I take a day off), my productivity always goes down.