The "natural state of man" is to work from sunrise to sunset to find enough to eat, and still sometimes going to sleep hungry. The hunter-gatherer lifestyle wore people out so they died at 30! The natural state of humans is to be active. The question in the modern world is what activity.
Incidentally, my experience of job sharing is that it doesn't scale. You get 2 or 3 people, they arrange the job share among themselves, everyone is blissfully happy. Then one leaves and how do you fill the position, when it was tailored for one specific individual's lifestyle? The other sharers then find themselves having to adapt again, and maybe that's incompatible with the other commitments they've picked up thanks to the arrangement. 9-5 is actually the least-worst option all things considered.
Incidentally, my experience of job sharing is that it doesn't scale. You get 2 or 3 people, they arrange the job share among themselves, everyone is blissfully happy. Then one leaves and how do you fill the position, when it was tailored for one specific individual's lifestyle? The other sharers then find themselves having to adapt again, and maybe that's incompatible with the other commitments they've picked up thanks to the arrangement. 9-5 is actually the least-worst option all things considered.