This claim is testable. Work hours have been shortened hundreds of times in dozens of industries over dozens of countries either by union contracts or by state

This claim is testable. Work hours have been shortened hundreds of times in dozens of industries over dozens of countries either by union contracts or by state laws. If your claim is correct, surely you would be able to back it up with some data. In fact if it were correct, the dozens of legislative assemblies across the world which keep shortening the working hours for various industries would know about it though the literature. Usually when work hours are shortened it comes with a guarantee of same pay and a clause for guaranteed compensation for overtime. If an industry relies on 40 hour work week the workers should then simply be paid more to compensate. If not then workers will enjoy the extra free time without any losses in pay.