That’s why this is a hot take 
I think you misunderstand me. I’m not saying the HD 6X0 is bad. Rather, I think it’s very good, and one of the best headphones we have for midrange. My gripe is around the fact that some folks put the HD 6X0 on such a pedestal, and consistently tell others the same, that nothing will ever touch it, even if something truly better does come out. As you said, it gets the midrange reasonably right. But reasonably right isn’t the same as “all other mids are bad” level of fanaticism. And ironically, most people probably aren’t even hearing the “perfect” HD 6X0 midrange that seems to be romanticized because of pad wear and variation. They’re hearing the reasonably right version.
For example, take the recent HD 505 or 550. Both of these headphones have midranges that are also reasonably right. But few people are out there banging the drum for these headphones. If there’s lots of people truly hungry for the HD 6X0 to be dethroned, the HD 505/550 would’ve been their answer. Yet other than the initial reviewer wave, I’ve seen quite little talk of them. And when you put them up against the HD 6X0, you’re likely to find someone say the HD 6X0 has better mids in some way. Or at least, lean towards the HD 6X0. But is that because the HD 6X0 is truly better? Or because years and years of people repeating that the HD 6X0 is the best that there’s an unconscious bias that’s been planted? To somewhat echo what Taron is saying - if the HD 505/550 existed 20 years ago and the HD 6X0 came out today, would it overtake the HD 505/550 and receive the same cult following?
The concepts behind the novelty explanation make sense, but I don’t think novelty is the right word. If anything, the sound of the HD 6X0 is novel to someone new to the hobby. For those outside the hobby, you would pretty much never have been exposed to the HD 6X0’s type of sound. Like, there is no scenario in which you would naturally have heard the HD 6X0’s midrange focused, closed-in sound without actually have ventured into the hobby before. The musical experiences people not into headphones/IEMs would have experienced would be either live concerts, playing instruments, or some mass consumer headphones like the ATH-M50x/Airpods Max/APP2. None of these are remotely close to how the HD 6X0 renders sound, with maybe the exception of the APP2. But even then, that’s a TWS IEM with lots of DSP and other effects going on which is also very different.
Saying novelty is also too dismissive of people who have been in the hobby for a long time, who’ve owned or tried all sorts of headphones or IEMs from Sennheiser to HiFiMan to Audeze, and MoonDrop to Campfire to 64 Audio, and yet still don’t love the HD 6X0 with that level of conviction. Novelty implies that once someone gets past the “beginner” stage and becomes an “experienced” audiophile, that everyone will return to the HD 6X0 and use that as their sole headphone. Which does happen, but not universally.
Lastly, the HD 6X0 is arguably colored in all other aspects outside of its midrange. It has little subbass extension, and its treble, which not as bad as say HiFiMan, is still not quite right either. And regardless of whether soundstage and imaging are actually real in headphones, the perception of being music being closed-in is a coloration unto itself. This isn’t to single out the HD 6X0 for how “intimate” it feels, but rather to point that all headphones/IEMs are innately colored at varying degrees because they simply aren’t like speakers.