Odds of Hanging On
Let's put this into perspective. As time marches on, roads, bridges, and other infrastructure continue to deteriorate. Cities, towns, and governments will repair them as long as possible; upgrade them with new technologies; and replace them as a last (and expensive) resort. All of these projects require professional surveying work. That means continued opportunities for you.
Of course, keeping yourself healthy increases your odds of hanging onto your surveying career. No lectures here; just note that you'll find it pretty difficult to handle this extremely physical job if you've packed on the pounds and spend most of your free time plastered in front of a video screen. Make your own choices.