You all are giving wonderful examples of exactly why WotC won't actually DO a new Greyhawk setting book. Because they don't want to have to deal with all this crap about what should or shouldn't be in it.
That's why they stick with the Realms and Eberron, because both of them explicitly DO accept anything and everything and can find a place for it, without folks saying "X shouldn't be here!". So it doesn't matter what the new D&D edition has introduced that is different from the previous ones... new players and old players alike can use what they have and find a place for it.
But if older Greyhawk players can't accept the idea of the current 5E races all having a place in the setting and will fight tooth and nail over it... then why should WotC even bother? Those that want to play in Greyhawk can buy the older setting material off of DMs Guild and then bring in whatever 5E rules and mechanics they want, leaving out the rest.
This has been my thought on this for a while, and I know it drives the Setting Purists nuts, but why would WotC waste resources on a setting that doesn't support their vision of D&D?
Right now, WotC has supported (in some form or another) 4.5 settings: Forgotten Realms, Ravenloft, Ravnica, Eberron, Saltmarsh/Greyhawk. Each has been supported differently; ranging from a campaign guide to just enough material to run a small sandbox. That being said; I think we can attempt to draw conclusions on what WotC thinks of their settings.
First, they prefer a light touch. They aren't in the business of uberdetailed setting bibles anymore. We have had only a small portion of each world detailed (Sword Coast, Saltmarsh, Barovia, District Ten, Sharn) detailed, and the rest left open or detailed lightly. Of course, Faerun has had some additional areas given greater detail in every module released (Underdark, Chult, Waterdeep, Baldur's Gate, etc) but even then, the detail each area has gotten pales to the massive tomes written during 2e or 3e. Ignoring that, WotC seems to prefer a "here is a starting local and a map; fill in what you like based on the sketch" model so far.
Second, they only change what is needed and do so sparingly. The most radical change was Ravnica (which removes most of the D&D races to replace them with MtG races), but beyond that, they haven't been in the market of saying "no." Barovians don't react to dragonborn or tieflings and worse than they do elves and dwarves (which is to say, they dislike but tolerate) Similarly, Curse of Strahd takes much of the magic restrictions and changes and chucks them out the window; keeping only the ban on planar travel/escape and a few minor tweaks to some spells. Certainly, not to the level previous Ravenloft settings changed magic. Saltmarsh has a tiefling NPC; Eberron has homes for post 3e races, etc.
Third, they aren't re-inventing the wheel. If Ravnica wasn't an excuse to port over hundreds of MtG spells, mana-based spellcasting, and planeswalkers, then I guarantee they aren't rewriting classes for Dark Sun, Dragonlance, or any other setting. They may add a few extra rules (such as updated item creation in Eberron, or defiling in Dark Sun) but I really think class changes will be done via subclass or not-at-all.
Fourth, WotC isn't afraid to add stuff. Dusk elves didn't exist in Barovia prior to 4e. Genasi live in Athas; the Elemental Evil Player's Guide explicitly says so. Half-elves were never explicitly stated to exist in Ravnica, but they are a playable race there. Again, we have a tiefling in Keoland.
Fifth, and this is important; WotC is not above retconning things. Strahd's origin was given some noticeable changes to fit with the updated Barovia. The tabaxi tribe of Chult are now the same as the tabaxi cat race. A fair amount of the gnomish deities in Tome of Foes got sex changes. "The way it was before" is not set in stone, merely a suggestion.
All of these trends lead me to think that WotC will continue to be fairly open when it comes to allowing new options in old settings. Its not going to be the floodgate that people want to hang as a strawman; there will be no ocean of aboleths in Dark Sun or warforged in Krynn, but I don't expect many hard-bans. I used to say none, but Ravnica showed they are open to some tinkering (at least at the racial level) to fit the setting. That said, I still think the main takeaways of WotC's setting strategy is akin to seasoning packs for D&D rather than strict diets; a way to flavor D&D in certain ways rather than rewriting it.