Well, since you quoted me:
Officially published D&D settings have one major goal: support the current version of the game. It can do that by providing lore, options, mechanics, or theme, but the setting is beholden to the main game, not the reverse. As such, a setting should look at what D&D is currently, and say "where does this fit?" (or "why can't this ever fit?") rather than be beholden to the era where it originated. Too many fans of classic D&D settings end up marrying it to the edition it came from (or was popular in) and thus the setting doesn't get to grow. The setting stagnates; it become an anachronism to a game whose classes and races have long since evolved past where the setting was. It becomes beholden to long lists of "canon" and "noncanon" sources, ban lists of options that didn't exist when the setting was first established, and increasingly complex house rules used to mimic the "feel" of editions that have long since passed by.
All that said, a Greyhawk can exist quite easily in 5e; Saltmarsh alone proves that. That Greyhawk though isn't going to look quite like Gary's version though. Too much time has passed, to many things have been added. You might be able to cut away the godawful Castle Greyhawk module, but it's a losing battle to fight against Tasha/Iggwilv, Vecna's divinity, or that dragonborn, tieflings, warlocks and sorcerers can't/shouldn't exist. The setting cannot be encased in amber and shielded from all change and also be relevant and supported by today's D&D.
What should happen, as with all settings, is a simple question must be asked: What is the essence of the setting? What makes it FEEL different than the other settings? Usually it's the genre, the tone, or "story" the setting is telling in its history, geography, and politics. Greyhawk captures a grittier, more mercenary tone of adventuring that focuses less on "save the world" heroics and more on "survive in the face of insurmountable odds". That is a setting worth exploring; one not steeped in shiny knights and evil dragons but one of rogues, necromancers, and others just trying to survive. That essence isn't bound to the specific names of NPCs or the available character options, and it would make a fine counterpoint to settings where Good and Evil are much more clearly defined.
So yes, I can see a Greyhawk guide that distills that essence of the setting, including elements from Gary and from later authors, into a viable setting that supports and is supported by the 5e ruleset. This is not hard, but it is not going to be a faithful copy of the setting that existed in 1982. That setting needs to grow and change in order to thrive today. And if it cannot, then it should remain a relic of a bygone time, married to a certain era and edition, as a footnote in the long story of D&D.
Empires that refuse to change cease to be Empires for long.