A 7 day work week seems to be optimal for most. Specifically, the 5 days on / 2 off. Apparently, medical staff working 10 / 4 have issues, and attempts at running e.g. 7/3 also don't go over very well.
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.