The first of your points is the Hawthorne effect, and I agree that we should not conclude anything from short term productivity increases. But your second poin

The first of your points is the Hawthorne effect, and I agree that we should not conclude anything from short term productivity increases. But your second point is just absurd. If wanting to keep your shorter working hours turns out to be such a good incentive to work harder and smarter that it improves productivity on the long run, then how does this not prove that shorter hours improve productivity?