In 1934 people were trained on the job. This was as true in the WPA as it was outside.
What changed is not the newfound impossibility of doing that, just the reluctance of employers to pay for it and the willingness of the government to indulge their insatiable demand for cheap, pretrained labor.
there are many reasons to be against a job guarantee but the presumption that every potential job nowadays requires a 7 year degree is a particularly bad one.
The main difference from then, however, is that it is difficult to give each man a showel to dig and each woman a kid to care for this time.
People need qualifications to operate heavy machinery, know regulations, etc. - we are not in 1934 anymore.
As such we also don't need 30% og the population in the farming sector.