Personally I haven't found the peak fidelity of headphones to have improved much, what there is, is a lot more good headphones that are better realized. The baseline quality of headphones is at the best it's probably ever been. There were definitely more trash headphones in the past, but the peak fidelity of headphones and speakers hasn't seemed to of changed much as actual drivers have been a mature technology for a very long time. And in terms of other driver technologies, many old STAX electrostatics are still better than a good chunk of modern headphones. The biggest improvements in the future may come on the software side, such as DSP filters. Biggest improvements recently have been better utilization of the drivers, materials, better housing, tuning, etc. but this doesn't always translate into a better sound. Sometimes vintage headphones are just as good with a bit of proper dampening. What I consider the best headphone I own to my ears is 40 years old, it just has some mods on it to better realize it's potential. And some such as the R10 and original Orpheus have a sound that hasn't really been replicated.
This is a market that is now saturated with so many headphones and so many new releases that it's easy to overlook the greats with the new stuff around, the thing is the new stuff may not always be better. It's gotten hard to keep up with all the new releases compared to when I first got into this hobby, many major players now were in their infancy or haven't even gotten in the game yet.