So why now advocate for 6 or 7 day work week then? Because time off is worth a lot. And going from a 5 day work week to a 4 day work week is a 20% reduction of work days but a whopping 50% increase in weekend days.
Meanwhile the number of doctors and plumbers etc. is not a fixed constant but driven by demand. There are plenty of qualified people currently doing useless work or unemployed who could fill those roles with the right incentives.
It is, but someone still has to work to keep society going in those "time off days". Doctors can't treat the same amount of patients in 4 days as in 5. Teachers can't squeeze the curriculum in 4 days. Customer support can't squeeze everyone in 4 days. Etc.
So how will you increase the number of doctors and other staff to work in the days the existing staff are off?
You can't just magic more workers out of thin air, especially now when we're already in a labor shortage in many fields like healthcare with everyone already working 5 days/week and with fertility declining in the west.
Meanwhile the number of doctors and plumbers etc. is not a fixed constant but driven by demand. There are plenty of qualified people currently doing useless work or unemployed who could fill those roles with the right incentives.