All of them. There's a huge problem with folks not getting into the trades. HVAC, electrical, framing, millwork, machining, masonry, plumbing, welding, etc etc. I live in a small town. A young local guy started up a small plumbing company about five years ago with contemporary thinking, branding, marketing, etc. He's a super sweet dude that volunteers a lot of time, runs some community facebook groups, etc. Today he's got 50 employees operating out of two locations.
Another question from me: what do you make of the "its backbreaking work" argument? That is, trades are more precarious as you age. Its easier to sit in a chair at 50 than to hunch and squat and crawl.
You sit in a chair until 50, you'll have a heart attack. You do physical work until 50, you'll have an injured spine. Take your pick. Wage labor wasn't designed to be healthy.
It's definitely a factor, some more than others. Unfortunately because the labor pool isn't replenishing like it used to, tradesmen are doing the heavy stuff much later in their career and taking larger tolls on their bodies.
Shit's there for the taking.