Disagree. If you're earning enough for a decent quality of life working 4 days, there is no real reason to work 5 days.
Where I live many people choose 20% earnings penalty, because 80% is enough for all needs.
And we remain very competitive on the global scene, as a society. We ensure we make the most of those 80%.
My broader point is that anyone who can work 4 days instead of 5 and earn the same is an incredibly privileged position. And many others will be nipping at their heels. If you’ve built that moat to allow you to do that sustainably, well done. More often than not, over the long run human drive at a societal level will create competition and this won’t be sustainable.