Why would anyone assume that everyone (in the US and other wealthy nations in particular) is economically viable as an employee at prevailing wage rates?

Add in increasing regulatory and financial burdens to employers. Add in an economy increasingly based on information, where information coordination is more and more a relevant and limiting factor. Add in technology rapidly making people with meager skills obsolete.

Even without the add ins, management and coordination have financial costs and opportunity costs that a worker's productivity may not overcome.