I think we've built companies and cultures that are incompatible with long-term employment and happiness.
Anyone who joins a company can crank full 8+ hours a day for a while to establish themselves (and a reputation).
The "problem" is, as people establish themselves, the problem domain becomes less exciting. There's less urgency to crank indefinitely. They settle into a pattern that involves fewer hours, though those hours are more productive because they know the ropes.
There is a sweet spot where someone knows enough to be productive but isn't yet complacent. This is the spot that every employer dreams of: employees cranking, full speed, productively, for 8 hours.
It's just not sustainable. You can fire people and try and keep turning over staff such that everyone stays in that sweet spot, but you'll eventually end up with a different sort of headache when your staff has no organizational memory for why decisions were made. The people who built things and have the long-term visions have left, and those who pick up the torch try will never have the same big-picture in their head.
The challenging bit is how do you separate someone who works 3h a day because that's all they can sustain (and they're just being realistic), and those who work 3h a day, could work more, but chooses not to? I'm not sure you want to force either out, but can you incentivize the latter to produce more?