Odds of Hanging On
Your odds of hanging on are very, very good. People don't like to jump from GP to GP if there isn't a good reason to do so, so once you've begun to build up a large bank of regular patients, you're going to be set. Just don't do anything weird.
Your odds get even better when you consider how unattractive the field of general practice is becoming these days (because of the relatively small pay), which means you'll be facing even less competition in the future. No young punks coming up through the ranks to push out all the old geezers. This isn't Wall Street.