A few years ago, we had some remodeling work done on our house. A union plumber came in on a Saturday to do the plumbing portion for cash. He told me that his most recent union-sponsored continuing education class was a call-to-action on the shortage of young plumbers. The union was worried that it could not find enough young people interested in the trade! I had always regarded the unions as exclusionary, but they're now worried about their own survival due to attrition (well, at least the Cleveland-area plumbers union).

This same guy noted that the real concern wasn't residential plumbing. This work is fairly low-skilled. The issue is that large commercial buildings require much skill and experience. Perhaps a solution would be to stratify the definition of "plumber" to allow those with a lower-level license to do easy work. Those with the upper-level licenses could do the more complicated stuff.