Businesses don't want to hire people at 20 hours a week if they are willing to work 40. And this isn't black and white - we have been in transition to mental labor automation for the past 50 years. We have just been replacing desirable industry jobs with service jobs that people hate with a passion, as wealth inequality grows and those dropped out of economic potential switch to basically waiting on those that have not yet fallen.
UBI I would argue is much more desirable, if you were going to try to legally force 20 hour work weeks. UBI actually lets you absolve a lot of historical regulation that holds industries and growth back, that was all established as an attempt to protect the poor and uneducated from exploitation. When everyone has an easily accessible minimum standard of living, regulations in the workplace can be retired to enable more innovation since the workers can finally negotiate evenly with the employer rather than being exploited to survive. It also lets those workers with the ethic to do so to spend as much time as they want making as much money as they want, knowing they won't starve or go homeless when they stop.
Either you keep cracking up the extreme regulatory machine currently asphyxiating the American corporate sphere (ever wonder why all the innovation is in software? It isn't just because its new, it is also because relative to almost everything else it is extremely unregulated) or you just use increased taxes on unfettered growth to provide for the masses so that the unleashed potential doesn't destroy lives.