I think Dank Pods explained this the best. High quality audio has existed for decades, they aren't making huge innovations in sound quality anymore, they are making innovations to comfort and the listening experience. Modern headphones sound as good as old headphones, but probably have features that the previous ones didn't. I guess innovations have been made to try new things in the pursuit of better sound. I know the new Sennheiser IE 600 are 3D printed and are able to use a very interesting internal housing design which improved the sound, and that design is only possible because of 3D printing. Also look how far bluetooth headphones have come, I know for a fact modern Bluetooth headphones are better than Bluetooth headphones from 6 years ago.

Also I see this with phones as well, they have gotten so good that there is no additional point to improve. Around 2020 all smartphone had realy good cameras, from cheap phones to expensive phones, Android or Apple, every phone has an amazing camera, so now in smartphone ads it's features. Samsung talks about their 8K camera, their 100x zoom, Google talks about their unblur feature and object eraser, even Apple talks about their Adobe HDR and filmmaker modes. They all know saying we have a good camera isn't enough anymore. And in terms of hardware there has also been a limit reached. The last 3 years of Samsung and iphones have had no changes, the phones are basically the same as each other and there is no reason to upgrade to a new one. And this is someone know knows and cares a lot about this and all the small details involved. That's why people care so much about folding phones, it's something genuinely new and exciting.