Try 25 years. The Sennheiser HD600 is still considered great.
I will say that there's a way better selection of great headphones now than pretty much ever, but things largely haven't changed. Planar headphones were the biggest thing to happen since then and they're not necessarily better than old dynamic drivers, just better at different things.
At the end of the day, a headphone just turns squiggly electric waves into a medium that wiggles and pushes air in ways that make noise. Tech advances don't really do much to change that basic concept.