Lots of good stuff there and here in the thread.
My favorite paragraph -
A class has to be progressive, carefully allowing its boat to evolve with the times. Drastic changes are just as bad as none at all. If the modernization is too drastic, you obsolete all the existing boats. On the other hand, when development stagnates, there is no incentive for sailors to purchase new boats and upgrade their equipment. If people aren't buying boats, then the builders get out of the business.
Exactly the philosophy of a formula class.
I keep thinking about the Hobie 16 when this question comes up. It is undoubtedly one of the most successful one design classes in all of the sailing community. However, I think the equation for a successful class has changed over time. The Hobie 16 was born in the "hayday" of catamaran sailing and emerged the clear winner and I think a lot of it's current popularity is still due to the size of the class that is a result from it's early beginnings.
Today's emerging classes have a different struggle in that the base of active sailors who can actually purchase a new boat is considerably smaller and the boats have to target a more specific need. This is why we've seen a lot of growth in the formula classes because they have found a way to meet several various, and previously conflicting, sailor needs and desires. Versus 1970, there is certainly a different balance now with regard to the equilibrium between development and stagnation and I think this balance changes over time with various influences in our social practices and economics. This is one of the reasons you see some classes come and go. At the same time, it is also very difficult to take a particular class and modify it's balance between the two without alienating the base of sailors that are already established. I don't believe you could ever take the Hobie 16 and open it up for increased development without killing the class. Likewise, I don't think you could take the A-class or the F18 fleet and lock down future development without killing it. It's a ballet to build a class and keep it alive that depends largely on society.