Outside of production boats many new boats are built in small yards from designs that came out decades ago and are just the pick of the proven offerings that would be very hard to improve on. They come from a variety of popular design offices. I can’t see that changing much for that type of boat which is still popular in many places. Like here.
The turning point to 'more modern' was a more modern underbody which started what maybe 50 years ago. like the gains to larger boats changing from a full to a 3/4 keel and skeg rudder. It's still hard to better that underbody shape for a larger boat with a limited draft and low aspect vortex lift keel. Except for minor gains such as maybe adding some balance to the rudder to reduce steering gear loads. But apart from that what would you change given a fixed displacement and a target Cp for what you call a modern design ?
I have redesigned heavier full keel boats from well known designers like Charles Wittholz to good effect with a fin keel and a balanced rudder and they have certainly made better boats than the original. But there are many good tried older designs that are extant already. People are still having them built or building them themselves.
I know that. Amateurs and low budget builders are still using old designs but that designs do not represent the state of the art and they are just building from old plans because they are cheap and cannot afford a brand new plan from a major NA. I have been there, I mean planning to have my one boat building and I was doing just that, trying to improve over an existent affordable design with already some years. Not the top of the crop, I am afraid but the best I could afford.
Mass production shipyards or even smaller shipyards that produce boats have the money to command true contemporary designs of all types from the best NA, designs that represent the state of the art.
I agree with you that the gains from the last generation (10 years ago) from the next generation is not big but still meaningful. But each generation shows improvements over the last and the difference from a contemporary design (of any type) with a 30 year's old design is huge, not only in performance but in interior comfort,available space and the living quality of the space.
We can notice better that evolution on mass produced boats because the competition there is huge and they are forced to make always better boats to keep up with the competition and for that they command work from the best NA, even if that is expensive. They have not a choice except to have the better designed boat they can get.
They normally change the hull of a given model each 7 years or so and in between they make a MKII with the same hull. They change the cabin design and the interior, improving the rigging layout. I don't know of any European brand whose actual model was worst than the previous one, in comfort, performance or easiness of sail. They have to do so or they go out of business.
There is not this kind of pressure on the amateur or semi amateur boat building sector and there the priorities are others like having a boat easy to build, affordable but not too old plans and an overall not bad sailing.