My guess is the problem is simply this:
* The pros that need a closed back are happy with the HD25 or HD280pro, and see no reason to spend more
* Sennhesier can't make a closed variant sound good enough where both audiophiles will want to buy it at $500 and Sennheiser makes a profit off of it
You see this in the HD820 reviews. "Doesn't sound as open and neutral as the HD800S, and the HD800S is $700 less! Obviously the HD800S is the smarter choice!" Yeah OK, unless you actually WANT a closed back. But thats not how many audiophiles think. Many think best sound #1 and everything else doesn't matter, so the idea of paying $700 more for potentially the same or worse sound seems idiotic - when in reality that is simply what it costs for all of the extra R&D and technology it takes to make a closed back sound as good as an open back. The fact the HD820 sounds anywhere near an HD800S is an engineering miracle, frankly. But it got mixed reviews anyway (in fairness there were a few intelligently thought out ones, though, that are worth reviewing)
I had the HD380pro a while back, it was sennheisers flagship closed back at the time. It sounded like a HD25 on steroids. I didn't like it and wouldn't buy it again. And thats probably why Senn doesn't feel a $500 closed back market is worth pursuing, pros not interested and won't sound good enough for audiophiles. You never know tho...
I would like to have a pair of 820s but I don't have the money right now. A closed pair of cans in the same price range as a new pair of 600s would fill in a gap. There are numerous closed headphones in the 500 dollar category, I think there's room for Sennheiser and they should be able to make it happen with whatever changes are necessary, and not charge 2500 dollars for it. I just bought a pair of DT1770s for around 500 dollars, so...
Neumann NDH20. The new closed-back studio headphone in HD650 price range manufactured and sold by Sennheiser since 2019 but as the first headphone under Neumann branding 18 years after Neumann GmbH (primarily studio microphones and monitors) was acquired by Sennheiser. Design is allegedly based on the failed Sennheiser HD630VB -can't confirm if it's true. Reviews of NDH20 from pro community seem to be exemplary.