However the housing stock grows, the remaining stock needs maintenance anyway.

So, well, indeed, a good plumber should not see extremely lean years.

(Anecdotally, my son works as a building systems maintenance engineer, without a college diploma. Of course the market for such jobs ebbs and flows, but seems to never dry up: city buildings still need their elevators, HVAC, fire alarm, access control, etc systems working, no matter what.)