The essential problem is that technical improvement does not necessarily translate into better enjoyment of music. This goes back a long way (I'm going to pick on Sennheiser because they should have known better). The HD-650 was a technical improvement in terms of speed and detailing over the HD-600. The HD-650 became one of the de facto reference points that almost everyone was able to relate to. OTOH, it could never achieve the tonality of the HD-600. To this day, I would listen to the HD-600 over the HD-650. The technical milestones of the HD-800 are well known, and it also became a de facto standard. However, for listening to music, IMO it's bottom of the barrel. A toy to be used to show what headphones can do, but not a device for serious music listening. I have not heard later iterations to see if the flaws have been corrected somewhere along the line.
To this day, the Sennheiser HE90 from 1989 or so with a good amp powering it sounds better than 99% of new headphones that I've heard. The big rush of planars that has hit us in recent years may well be an attempt to approach what a top electrostatic headphone can do without the need for a top electrostatic amplifier. Where I've heard big advances is in the $400 to $1200 price range. This appear to me to be where technical advancements in recent years have pushed sound a lot. Fostex and Focal dynamic headphones, Oppo and Quad planars all punch above their weight (although Oppo is out of out of the game and I don't know about Quad). I have yet to meet a mainstream planar that I love, and my ears are getting old enough that I'm not sure it's worth the effort to keep trying.
