If 'custodial engineers' were to drop everything and become programmers, who'd do the dirty work that they do?
Nobody is talking about re-training people that are usefully employed. They are talking about re-training people whose jobs are disappearing. Robotics is advancing very rapidly. Jobs for unskilled people have been disappearing for decades, but the past is nothing compared to the avalanche of disappearing jobs that may soon be coming. History shows that, in the long term, economies adjust and everyone benefits from productivity improvements. But the short term transition can be brutal.
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It is essential that we keep the timelines straight on how jobs are lost, and then eventually regained in a true Industrial Revolution. We are currently in what should be called the Cybernetic Revolution, the only true successor to the original IR in terms of its effects on employment.
In the original IR there were rapid losses of employment (starting in textiles) as factories went up starting around 1780. Optimists, who prate about how 'the IR really wasn't so bad' argue that by 1840 the average wage had risen to finally exceed pre-levels. As with today, talking about average wages hides the extent of poverty with a society, but more importantly it ignores the fact that the gap between 1780 and 1840 is sixty years, and other more systematic analyses pretty much keep this same gap for the employment picture turn-around, though shifting the dates of both start and end forward slightly. This means the typical worker rendered a pauper in mid-life by the start of the IR never benefited, their children never benefited, their grand-children rarely benefited, it was only their great-grand-children that found ready work at good wages!
The promise that eventually the economy will adapt and replace the lost jobs is one that won't be seen for a few generations. We need to have policies in place now, as the jobs vanish, to keep the workers and their families from ending up in poverty, and these policies will need to be maintained and updated for several decades to come.
It is notable that a quick perusal of conservative policy sites (National Review, etc.) for suggestions on how to deal with this problem of inevitable long-term unemployment find by far the most common is to suggest that job salaries be subsidized by the government to create employment. The really aren't any other alternatives that might conceivably work - only government spending to stimulate the economy can step in. (But how taxes should be raised to finance this is never discussed.).