Hanging on depends on several variables. First, diving is a demanding physical activity, and that's especially the case as an instructor. You've got to keep a physical fitness level that means you can handle yourself in challenging diving conditions, plus you can bail out your students who get into trouble. This includes the lady who inadvertently drifted down to 130 feet without realizing it, or the guy who didn't pay attention to the current and has surfaced half a mile down from the dive boat. Yes, that means you've got to swim half a mile against the current with a very tired student. Ready for that?
Okay, let's assume you can handle the physical demands. Now remember you've got to keep current on the latest dive safety protocols, maintain your professional certifications, and keep up with the dive industry as a whole.
Now we get to that nasty little variable called money, which results from the opportunities available to you. For starters, let's say PADI training facilities pretty much dominate your region. That means your NAUI instructor certification probably won't be of much value unless you take some additional training to meet the PADI specs. Next, let's assume your area's dive shops are adding more specialty diving courses to their programs. Have you gotten the additional certifications to teach them? If not, you might be relegated to selling wet suit and regulator combos and conducting Open Water Diver classes a few times a year. In short, the key to your continued opportunities is being ready for them, and knowing enough about the dive industry to predict which way the winds will blow...or the bubbles will drift.