What you're talking about is building the first adventures, not worldbuilding. For worldbuilding, you need to go deeper than that. "Put the village of Hommlet here" isn't helpful. Why put the village in this location? How much detail and depth should this first location have? How much is too much and how much is too little? How big an area should you map out? What relation does this village have to the big cities around it? Is this important for the adventures you have in mind? Do you want to include every species in your world, or just some of them? What do they think of each other? Do some classes have a particular meaning in your world? How important is alignment for your setting? What sort of evil forces do you have in your setting? Where on the realistic-to-fantastic scale do you want to go?
Again, those are adventure building tips--which are, in fact, very helpful!--but they're not worldbuilding tips.
Well, to an extent... adventure building and world building are the same thing. For example, the how much is too much detail and how much depth is too much depth are key for both. And the answer for this chapter is going to be less detail and less depth, by sheer necessity. Again, we are likely looking at 30 to 50 pages, max. There is only so much you can do with that, when you have 48 subclasses, dozens of species, multiple religions, a few planar powers, and then politics.
For an example, I very much doubt that we are going to get much discussion on how hags feel about dragons, or how dragons feel about vampires, unless it is something unique and interesting.
Also, I do want to point out, we don't have the book, or the chapter yet. I can only make wild guesses about how they are going to start. Will they start with a blank terrain map, then point out civilizations built based on that terrain? Will they start with the gods and churches and have that lead into politics and trade routes? Will they talk about the various species and their connections first? I don't know. Any of those COULD work.
There are plenty of DMs who build a world that exists only so far as the adventure needs it to. And there are plenty of DMs who build a world first and the adventures later. Neither of those is superior to the others.
I've never even attempted to claim one is better than the other, so you can cut that line of argument off at the root.
But my question to you is, why is it so important that an existing world be used for this, rather than creating a new one "with" the players?
Why is it important or why is it useful?
1) It allows for the WoTC team to connect to the heritage of the game for the 50th anniversary. Yes, they could create something new, but by using something old, they acknowledge their past. This is important to them.
2) By using an existing setting, they reduce the workload. Creating a new setting from scratch isn't difficult, but it does take a lot of effort that the team may not have the time for since this is part of all the other core books they are writing. It may be that they decided that creating a new setting is best done with a new setting book, that they can devote their entire attention to.
3) Since this is the example setting in a core book, it needs to include everything in the core books, monster manual and PHB included. Creating a new setting where the Sun is an ancient dragon at war with vampire armies that come from the moon would be rather strange, since the Monster Manual won't talk about any of that. So, by necessity the setting in the core book will need to be slightly more "generic DnD". Since we already HAVE settings that are generic DnD, it makes sense to use one of them, instead of creating a third or fourth.
4) Many of the people who are writing these settings likely have many of their own creative ideas. Chris Perkins, for example, made a really fascinating dragon-island world over a decade and a half a ago. He may not WANT his ideas in the DMG, where it is owned by the company, and out of his creative control. Creatives get frustrated with things like losing control of their creations. A new setting, exclusively meant for an example chapter, might not be something one of the designers wants to put into the book and have stripped of their name and input.
5) Since this is an existing setting, that will then be available for use on DMsGuild, there is very little additional support that will be required to be published. You do not need to create additional adventures specifically set in the setting, you don't need a setting book, you don't need to create more, because the setting exists. You still CAN, and adventures have already been including Greyhawk as an option for the published adventures, but a new setting would REQUIRE those products and restrict what they can publish in the meanwhile.
Now, I think your real question is more in line we "why shouldn't we use a new setting" but you also keep giving off this feeling that using an old setting has some downsides. Frankly though... I don't think it does. Because the thing you are worried about? This idea that the old setting is going to be useless to new players unless they fill in dozens of pages of existing lore... I don't think that's needed. Yes, Greyhawk is already created, but they can present the chapter as though it ISN'T already created. The actual process of creating the examples and explaining the process is actually identical whether you are starting with a blank page, or a filled in page. Remember, this isn't like a live stream video, it is text. The writers would need to write a new setting, then go back and create the examples and the explain the process, because they can't type both things at the same time. They are just skipping the creation step, which has benefits.
Ain't nothin' but a heartache!
