20-Year Prospect
It's looking pretty harsh out there, twenty years from now. The job market is increasing at a slower rate than most other fields, so you'll have to be super on-it when it comes to new engineering technology in order to stay ahead of the curve. You should be fine so long as you do your research and choose the right field to get into.
Fortunately, while some mechanical engineering industries are in decline, some are actually doing pretty great. This just means you don't get to be quite as choosy about what you actually want to be doing. You may have to settle for oil and gas extraction jobs, for example, to pay the bills when you'd rather be building fight-bots.
But don't worry―the way technology is advancing, there are probably plenty of cool new fields waiting to be developed (hovercars maybe?) (source).
Nothing's for sure, but this job will definitely be around in some shape or form in the next twenty years. Maybe you'll even be the one of the people who does some of that shaping.