Odds of Hanging On
If you're a contractor, things are always going to be a little stressful. They'll be most stressful in the beginning, though, when you're constantly hustling for new clients. If you can stick it out, it won't be more than a few years before you'll start picking up some regular clients. After that, all you'll have to do to stay at the top of your game is keep up with the technology and always be ready to learn a new skill.
This career is based too much in physical labor and know-how for you to get pushed out of the field by some young upstart who's better at social media campaigning than you are. So you don't need to worry about some kind of tech disruption, but you'll definitely want to keep up with the latest mechanical developments in the field. And avoid getting injured. Not only does it hurt, it could put you out of work.