Odds of Hanging On
The market for a good ophthalmologist is there and will always be there. Even if times are tough and you open a practice that fails, odds are you'll be able to find yourself another job soon.
However, you do need to stay in good health. If you have a stroke, develop Parkinson's or a neurological ailment, you won't be able to operate. You need steady hands and cool, calm nerves of steel to do delicate eye surgeries.
On the other hand, you can always go into ocular research or get a sub-specialty where operating isn't involved.