I've found it to be the opposite, actually. It only gets messy if you have people who refuse to compromise.
Different experiences. I recently was trying to world-build with a group from my old job. It was like pulling teeth to get anyone to decide anything. Which made it hard for me unless I wanted to start stomping all over just making decisions for the group. And we were working in a "each person gets their own area of the continent" not a "each person covers everything at the same time, because we are all editing it for our job."
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.
It is the exact same question for a new setting though. Where do you start? A new location? The gods? The Theme and Flavor? The Species? This is the exact same formula, just without answers after the equal sign.
Actually, a good analogy here might be character creation. Where do you start? I've had people start with their background, others with their subclass, still others with a weapon or with rolling their stats. The answer is you start where you start. It is perfectly acceptable for the chapter to start with whatever they decide to start with, then expand or focus in as they desire.
Maybe they do Hommlet, then Greyhawk the city, then a few surrounding nations and the various factions, and they have covered a fairly decent sized portion of the map, while giving the new DM the tools to build on every level of that map.
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.
I mean, I can't tell you that your opinion is wrong. At a certain level, I see the two approaches (new setting or existing setting) as fundamentally identical for the purposes of teaching. Because you need to build the new setting before showing how to build it, and therefore from the author's perspective, both settings "exist" while you write the chapter.
But Greyhawk, as you said, CAN do it, and it has advantages that WotC wants to leverage for this 50th anniversary. As long as they write the teaching chapter well, I won't begrudge them that.
