Odds of Hanging On
To be perfectly honest, the "hanging on" part is totally up to you. If you do your homework, know how to serve your clients, and work hard, you're in for as long as you please. This is a growing field, so there's no reason to think that it'll go the way of the fax machine, flip phone, or VCR. Energy is a utility almost everyone needs, so you can expect that any changes to the delivery systems won't mess with your job security.
Beyond that, the advent and eventual implementation of newer, more efficient energy sources should make brokering good energy deals even more necessary. So, if you find that you enjoy the work (and are good at it, of course), there's no reason you can't have a long, successful career helping people stay plugged in.