Apart from "packing donuts" type jobs, most jobs require some degree of knowledge and skill. Some jobs have higher knowledge half-life than others. While IT has notoriously short knowledge half-lifes, construction has rather long half lifes.
Suppose that we train x+y construction workers a year, x work in the industry until retirement, y - retrain to different profession. This means that there are y*years dormant specialists that can be brought back to industry.
Suppose that we train x+y construction workers a year, x work in the industry until retirement, y - retrain to different profession. This means that there are y*years dormant specialists that can be brought back to industry.