20-Year Prospect
Millwright employment rates are growing seven percent faster than the average American job right now (source). That may not sound like a lot, but trust us, it is. Most fields would kill to be growing seven percent faster than the average American job.
Part of the reason the field is growing so fast is because the Baby Boomers are retiring, so the industry needs competent replacements. Plus, more and more workplaces are using new technology, so the ability to install, disassemble, and repair it is more marketable (and profitable) than ever.
As long as you keep up your skills and can work on the new technology that's rapidly replacing the old stuff, you're going to be millwrightin' for plenty more than twenty years.