[tigen]

"Most people" don't get to choose, they're stuck with 40. (Or more, for some salaried positions.) The idea is generally "all of your time".

In principle they can just not do those particular jobs. In practice most "careers" expect "full" time.

Some fields are inherently quite flexible, like some doctors/dentists can decide to not schedule anything for a week.

In the linked comment JC said "I am deeply appalled at the thought of trading away individual freedom of action and additional value in the world for that goal."

But the point is that freedom is ephemeral. The world has all sorts of incentives that promote race-to-the-bottom behaviors (cf. https://www.slatestarcodexabridged.com/Meditations-On-Moloch )

In the good ol' days at least a family might likely have the mom at home with the kids even if dad gave blood to Moloch every day.