Makes total sense to have a worker well rested and happy (40 hours does that sufficiently well). Your submission works if it is a task such as programming. A programmer can finish their week's tasks in 3 days or less and you'd be fine with that. That's not most programmers though.
Moreover, I have had other businesses where the time you pay for as an employer totally matters if you're to get an ROI.
If I need you for 8 hours a day and there's an amount we've agreed on as compensation, then that's exactly what I need.
It doesn't really work if for instance it is a restaurant and you need to have waiters, dishwashers and cooks round the clock as patrons visit your establishment.
If they'd rather work fewer hours and get less money, who am I to argue? I'm just saying they'll be poorer for it and contrary to what you posited, they won't be too happy about it either.
