Unemployment solved: 20-hour work week

While watching yet another presidential candidate promise how he would stimulate the economy to create more jobs, I found myself wondering why nobody is talking about an obvious solution: restricting the supply of labor by cutting the working hours.
 
Given the current productivity levels and 5 billion people of working age on the planet, it does not seem possible to give everyone a 40-hours/week job doing something useful.  There is a precedent for reducing working hours.  We went from 70 or more to 40 a while ago.  We also eliminated child labor.  Was it a coincidence that real wages went up and the middle class got created since then? 

Why is 20-hour work week a bad idea, or a good one?

>>16
Pretty sure that's a text book no true scotsman fallacy. There is quite an extreme difference between 20 and 40, not sure why you're bothering to debate this.

You could be arguing from a more fluid perspective, looking at the influences and trends of labor laws like ending child labor and reducing work hours, but you're just demanding one arbitrary static inflexible change.