That's only if a 40 hour week is the optimum. Since no one is really working 8 hours, 5 days a week, that's doubtful.

The 40-hour workweek is a direct function of (1) that it's advantageous to have the fewest employees possible, because each additional employee has overhead costs; (2) you can't make employees work more than 8 hours/day (a number that was arrived at through significant strife and is unlikely to change substantially); (3) everyone gets two days off in a 7-day period.

The 40-hour workweek isn't an arbitrary quantity, it's an empirically derived quantity. And the process of deriving it literally involved people killing each other. I don't see it changing very soon, except in edge cases.