Ah, another advantage I forgot. Since there are going to be multiple editors going over the chapter, having a single, solid concept with clearly defined edges is much easier. I've tried world-building with a team, it gets messy fast.
I've found it to be the opposite, actually. It only gets messy if you have people who refuse to compromise.
This doesn't make sense to me. People who are going to compare their homebrew to official settings are going to do so anyways. I have never read a world-building chapter in the DMG focused on Greyhawk... but I compare my work to Eberron constantly. Or to settings from fantasy novels. Also, you think a new setting made by professional game designers and creatives won't STILL cause people to look at their own work and feel inadequate? It will, trust me.
Perhaps,
but. When you have a ginormous existing setting, it doesn't feel like there's a beginning--especially when you compare to a setting like Eberron where the politics and religions cause the entire world to be connected in some way. (This would be less so in a setting like Greyhawk, which seems to have been made more piecemeal--although I admit I could be wrong about that; I don't know that much about Greyhawk's history.) If, as people have suggested, the goal is to start small--here is your Village of Hommlet, here's the surrounding lands, here's the point where the first adventure will take place, that's all you need--then using a tiny point on a preexisting map is going to feel very inadequate.
So let's say you take Greyhawk. Where
exactly do you start? A specific location? The gods? The setting's theme and flavor?
Or the species? There's been some talk on this thread about how to get the newer species into Greyhawk, with some people saying it's really easy and others saying that it goes against the setting's flavor. You definitely don't have to worry about that if you're making a new setting, because you can add them all in at the start.
Now, this isn't to say that the chapter shouldn't include things like "every first draft sucks, it is okay" or "even the greatest settings started with a single page of notes, size comes with time, don't stress about it". Those are great things to include and I would relish their inclusion... but I don't understand why you think the chapter focusing on Greyhawk means it cannot do that, but if they made Bodal the new setting TM they can totally say those things. I think you are feeling more like this chapter is going to be "look at Greyhawk, the wonderful setting" and less "here is how to world-build using examples" and I don't get why you think that is the case.
I'm not saying that a chapter focusing on Greyhawk
can't do that. I just think it would end up being better if they built a new world.
Plus, a lot of people have been clamoring for a new setting and this would be a good introduction to one. If it turns out that people like the setting that was produced in the book, then they put out a setting book for it. If people ended up being meh about it, then they don't.