The answer is that no one really knows how long a fiberglass boat will last. When I was researching boats recently, I decided that condition was the most important consideration. I bought a 45 year old boat that surveyed very good. It was designed for cruising and heavily built.
My intent is that at the end of 20 years the boat will be in better shape than today, and my costs should still be less than they would be if I had bought a new boat.
I feel an obligation to her to see that she gets everything she needs to continue a great sailing life. She has earned the right to be cared for.