I was just giving my two cents about why "undesirable" jobs usually offer low wages.
By the way, I see your point with devaluation. Personally, I'm in favor of offering everybody the possibility to study, without depending on the wealth of their parent.
Either the required training is enough of a disincentive to make the Job Undesirable... and pay high. Or it's not.
Required training is not likely to be so scarce when people can chose to train without risking starvation or surviving on 4 hours of sleep a night.
Which is the real problem most of those with "training" fear. It's devaluation when even poor people can get it without Herculean efforts.