I agree with most of your post, although I think your last paragraph has orthogonal arguments, perhaps you can help me understand.

1. If you work at a company with great purpose for humanity, then you can be satisfied with working 40 hours a week.

2. If you work at a company without great purpose for humanity, your job can and should be done in 4 hours a day.

Presumably, you're also saying a humanity-saving job can be done in 4 hours a day, but it's okay to feel good about more hours because of the mission's importance. If that's your argument, I don't think it holds, because people can find the intrinsic motivation to work on writing gum commercials just as well as they can building rocket ships. And presumably an ad agency would like to hire those people, such that the writers are just as motivated as rocket ship builders at SpaceX.

Or perhaps you're saying the motivation should be extrinsic and measured by what society deems is a good thing?