Should the standard full-time workweek be cut to four days with no pay cut?

I spent forty years watching management fads, and this one has the classic tells: self-selected participants, short timelines, and outcomes measured by the same consultants selling the idea. The UK trial ran six months; nobody knows what happens in year three when competitors who kept five days pull ahead. Microsoft Japan's famous 40% productivity jump in 2019 lasted one month, in August, when half the country is on holiday anyway. I am not against rest. I am against declaring victory from a pilot and legislating it. Let companies experiment, publish honest five-year data, and then we will talk about standards. Even sympathetic reviewers agree on the gap: What Works Wellbeing's review notes the trial had no real counterfactual, since participants volunteered and were compared only to their past selves.