20-Year Prospect
Human nature changes very little over time, which is good news for anyone seeking a career as a farrier. As long as people love horses and want to own them, farriers will be needed. There's no chance of outsourcing the job to other countries. This is a hands-on career. It requires a slightly finer touch than machines can allow, too. So long as you live in horse country, you'll be set for the next twenty years at least.
As long as your body doesn't give out, this job will be here for you.