Odds of Hanging On
Once you've landed a good job with a dentist, you're golden (amalgam).
The only thing you need to worry about is your hands, neck, and back holding up. Years of cleaning teeth may give you carpal tunnel or tendonitis.
Also, if your boss retires, you may have to look for another job. The new dentist is liable to have her own dental hygienist already. Still, you're the one with the loyal clients, so there's leverage there.