@MissLucyEyelesbarrow
I can see how it would work for some industries, but what about all the services that have to function 7 days a week - utilities, police, healthcare, transport, retail etc etc? It's no use having more productive people 4 days a week when you need people actually staffing the services 7 days a week.
If you reduce a full-time equivalent working week from 5 days out of 7 to 4 days out of 7, you increase your workforce costs by 20% at a stroke. Yes, those individuals may be more productive but that doesn't produce much benefit because you still need to cover the day that they are no longer working. A highly productive nurse Monday to Thursday is no use to the patient who needs care on a Friday.
Lots of sectors like social care are barely able to function on current funding. If you increase their workforce costs by 20%, they will go under.
Sure, it definitely is more complex than the headline and it might not be directly applicable in the same way in every sector. But I guess you also have to look at the bigger picture of how it plays out in wider society and the economy rather than solely at individual business/organisation level?
For instance, Scandinavian prisons are run completely differently to here (loss of freedom is the punishment not the prison conditions - so the facilities are humane and decent like anyone should live in, the activities and training and support are designed to genuinely rehabilitate not traumatise, and the staff are better trained and remunerated), which took a much greater level of investment. However the result is that re-offending rates have dropped, communities are safer, and they don't need as many prisons. All of which are great on a big picture level.
So perhaps, using nursing per your example, investing more in nurses who provide better care to their patients means a healthier population and fewer people needing hospital care for as long. Fewer people leaving hospital traumatised or with health problems caused by neglect. The population working healthier hours and lifestyles could also have a positive knock on effect in terms of demand on hospitals.
People will have more time to exercise, and to spend money on leisure/social activities, which stimulates the economy and provides more tax income (both from individuals and businesses).
More people with caring responsibilities would be able to work. Some people would end up less burnt out from caring.
Society and the economy as a whole would change.
I'm not qualified to comment on precisely how, but I do know it's more complex than "how much money do we have in the pot for the wages" .