Fiberglass is not an unknown material today. A lot of papers have been written and a lot of testing has been done to put numbers to this question of useful life span. Three studies immediately come to mind and they are the cutting of sections from a particular Coast Guard motor life boat every 10 years over the life of the boat, the cutting of sections from the fairing around the sail on a submarine also on a set schedule and the study done by Gibbs & Cox at the request of Owens Corning to determine the design properties and fatigue life of polyester laminates. In 40 years of testing the mechanical properties of the glass did not change in any of the tests on the motor lifeboat or submarine. And the testing done by Gibbs & Cox set a design value for stress cycles that translates into some huge number that meant you had to sail every day in above average wind and seas for over 70 years to see a change for a properly designed structure. Unfortunately I donít have the papers in front of me so I canít give you exact numbers but glass is going to be around for a long time.
The thing you want to pay attention to is the design and construction of the boat. A weak design will fail in just a few years and a suitable design has proven itself over 40 years and its still going strong. The debate should be about how to care for the glass and what constitutes a good design. The things like superficial crazing of the surface of the waterproofing of the underwater areas of the boat are things that need to be addressed. Buy a suitable boat for the sailing you do and take care of her and she will outlast you.
All the best,