You do realize that outside of a literal dictatorship, a prime minister can't enact such a change on her own, right?

In some scenarios. Some types of office work in particular, and not for everyone. Some people work best in short spurts, some do not. Those of us who qualify are incredibly privileged, as are the industries we work for.

Now try this with manufacturing, healthcare, etc. 24 hour weeks means that to get the same output, you have to hire nearly twice as many employees, at the same monthly cost per employee. There probably aren't enough qualified unemployed people around to do that, so it's easier to just let productivity drop (and do hospitals REALLY need so many people around 24/7? At nearly 2x the cost, perhaps not - can't just increase pricing to make up for the cost increase in a single-payer system).

Its not that simple

Even if people are more productive in less time, that doesn't mean you can earn the same money. Many businesses operate on "hours spend" per task. As an example, let's take hairdressers. If your hairdresser is now working only 6 hours instead of 8, that's a potential 25% loss in revenue. And they won't be magically faster now just because they work less. So if salary reduction is not an option, the only option is increasing prices.

What you're saying is fine for office jobs, but just not an option for most service jobs. Waiters aren't magically going to serve tables faster, cashiers won't make customers move faster, etc.

Depends on the situation. Suddenly going to 60% of the work load isn't suddenly going to make the country or it's people more productive. You'd need to hire more people to get the work done and Finland's unemployment rate is already at 13%. So this would be a pretty poor solution when it comes to economics for everyone, unless you pay people less because they are working less hours.

Productivity, but not production. They would still produce less than now, how much less is just not a simple ratio calculation.