Half the population is below average intelligence, and perhaps 25% (total guess) might be best suited to a manual or semi-skilled role. What are we going to do with them?
When similar things have happened previously, the once semi-skilled (eg) shipbuilder becomes a low wage driver or call centre worker. Many in an area didn't get those few roles, so became unemployed (and often depressed) for the rest of their careers. Some steel or shipbuilding towns haven't fully recovered even today.
Longer ago the farmhand could get any number of manual roles in the growing industries. Blacksmiths became car mechanics, fuel attendants and so on.
Also when it's only a number of regional cities affected, like with coal, steel or shipbuilding, it's much easier politically than when it's national.