The numbers suggest that we could live 1950-style lives working only eleven hours per week[0]. Instead, we inflate our consumption and keep working 40 hours, living in houses more than twice as large[1], commuting ever-longer distances[2], paying daily (or more than daily) for others to prepare food we could have prepared ourselves, and just generally choosing to live many times as large. We adapt so quickly that we don't even realize that we've nearly-quadrupled our lifestyles, but that's where the 30-hour work week went.
Instead, we inflate our consumption and keep working 40 hours
"We"? I'd happily work at software rates for 20 hours/week. And, yeah, I understand the fundamental shift in a lot of things before that's a practical option. I'm not even disagreeing with you, I think you're spot on. But those that have simple, inexpensive lifestyles still don't have the option of working less. So though even I consume more than I used to, I also try to shovel as much as I can into the retirement accounts.
Housing is the main cost, and it scales with average take-home income — which is why two-income-no-kids families aren’t as well off as one might expect from looking at single-income families from the 1960s. If you can somehow disregard housing as a cost, and have health insurance anywhere except the USA, 20 hours per week is fine.
I’d manage on 10 hours per week minimum wage in the UK, but that doesn’t really count because the UK government effectively subsidies everyone earning less than (I think) £24k/year with things like NHS healthcare and a functional police, fire brigade, and army.
Housing is the main cost, and it scales with average take-home income
Shows you what happens when you're well-ensconced in your own personal bubble. It didn't occur to me until you pointed it out that the reason I could easily live on 20 hours/week is partially due to the fact that our $650K house cost us a third of that, because we bought it twelve years ago. Living on 20 hours/week and making the payment on a $650K house might be a little tight. Fine, make it 30 hours/week. :-)
[0] http://groups.csail.mit.edu/mac/users/rauch/worktime/
[1] http://www.moyak.com/papers/house-sizes.html
[2] https://www.thoughtco.com/americans-commuting-over-100-hours...