> Nicholas Bloom, an economist at Stanford University, says it is doubtful most businesses can shed a fifth of the workweek and maintain productivity. “Whenever I talk to managers, they find the topic pretty insulting—they argue it implies they are completely wasting a day a week,” he said.

Yeah, that's the point. You shouldn't expect the same productivity. You should expect less. Just like in theory if we went back to a six day week we'd expect more productivity.

The change isn't expected to hold productivity, it's expected to make people's lives easier and take advantage of technological advantages. Go to a four day work week and you can expect the same productivity that you had 20 years ago at five days.

Nothing wrong with that, if reduced productivity is even true, but most studies refute that anyway.