Just as you say. Those who care, the stuff is available. WotC have no reason to do anything.Except for those of us that do care.
Fortunately all those earlier editions have plenty of ideas on how to handle running that setting.
The Ravenloft UAs where 1 year and 4 months ahead of publication. On that basis whatever book or boxed set the "Feywild" UAs are for will be sometime between July 2021 and March 2022. I think it's far more likely to be this year than next.
You'd need about 200 pages for a player's guide to introduce people to the setting. DMG would add secrets of the nations, new magic items, give additional information on the outlying regions of the setting like the hollow world/red coast/good kingdom etc. Then throw out some nation books, bundle them into books of about 300 pages compiling a few nations each. A monster manual would be 400 pages alone. It would be a dozen books easily at the end. You could take the opportunity to fix the setting's glaring continuity problems, rework the weaker gazetteers into a similar style as the others. Reset the timeline so it doesn't end up with the post wrath malaise that ruined large chunks of 2nd edition. It would take some work, but it could be done. Just give us the chance.Mystara, similar reasons, but also.... there is NO WAY they would be able to do Mystara, The Known World, justice in a single 256pg hardback. They'd need at least, by my estimate, about 1200 pages to even remotely do it justice...and that's being conservative! Mystara is made up of 13 Gazetteers, each with between 75 and 110 or so pages, plus Champions of Mystara (another 250+ pages), Dawn of the Emperors (230+ pages) and Wrath of the Immortals (another 230+ pages). Don't even mention the Creature Crucible books (more demihuman and nonhuman 'classes' to play) or the entirety of the Hollow World part of Mystara.
what is the function of greyhawk anymore?Despise the cartoon movie to have been a total flop Dragonlance is one of the best D&D worlds to be adapted into a multimedia franchise. I guess the first step will be not a setting, but an updated version of the original compilation of modules, with some new options, for example Theros Ironfeld as an artificier, or his sister as one of the PCs. And the kender personality to be softed.
I guess Greyhawk and Mystara are going to be rebooted, to allow more space for new elements, not only the new classes, races and monsters.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.