I'm not trolling here...just curious. My own mind tends to go in the direction of the parent, but, I also see how enforcement would be next to impossible.
If there are any problems about work, they are about making it more enjoyable, making good work more widely accessible and distributing the proceeds fairly.
Until that changes, I would not expect part-time work to happen for anyone but really high up managers where such a trade may actually make sense.
Also see the book "Two-Income Trap", it basically describes how households didn't become better off from making both partners work, because that just bid down the price of labor relative to housing and education. Everyone ended up in the same position as before, but now they work 2x more and cannot opt out. Also the non-working partner in the family provided a safety net because they could start working if things got rough, and now that safety net is gone.
Surely the extra people working produced more output (e.g. ~twice as many bugs fixed/year or ~twice as many new anti-cancer drugs/year). Supply of some things would remain constraint and their prices would be bidden up, but many others would have their supply increased and prices could even fall (e.g. due to capital costs spread over a larger number of units). We definitely have not ended up in the same position as before.
This would not really be a solution. All it would result in is more people taking work home, because job evaluations would still be based on how much progress one made on the primary project. To get ahead and get that next promotion, many people would still feel compelled to work extra (unreported) hours. I suspect a madate such as what you propose would simply push the long work-days into being unreported.
This is basically what happens in graduate school (in the US, at least). I just finished a PhD program; as part of our contrats students agreed to only work 20 hours per week. There is no way one could finish a dissertation working on 20 hours per week, so everyone worked longer hours. Sure, you could complain about how many hours you worked (and some people did), but that did not change the fact that only 10–20% of Astronomy PhDs get faculty jobs, so if you want that faculty job, you need to put in the hours to do great research, regardless of the mandated 20 hour limit.