Huh? There is a page devoted to the Great Ring in the 4e MotP, which also makes it fairly obvious how much of what game before continues into 4e (The Abyss, the Hells, Mt Celestia, Carceri, etc). Not to mention all the Planescape, faction-y stuff (and Sigil itself) popping up all over the 4e planar stuff.
What page is the stuff about the pre-4e cosmology? I don't doubt you but I'm not going to read the whole 4e MotP to find it. I do see a section on Sigil but I've played 3.X for many years and never even encountered word one of Sigil so that really doesn't matter to me.
However, compare the chapters of the book. 4e MotP:
1. Exploring the Planes
2. The Feywild
3. The Shadowfell
4. The Elemental Chaos
5. The Astral Sea
6. Monsters of the Planes
7. Planar Characters (which includes magic items)
Vs. 3e MotP:
1. Nature of the Planes
2. Connecting the Planes
3. Characters and Magic
4. The Material Plane
5. The Transitive Planes
6. The Inner Planes
7. The Outer Planes
8. Demiplanes (which includes non-standard planes)
But it wasn't part of B/X. Nor was it part of Oriental Adventures (which assumed a Celesital Bureaucracy). Nor Krynn (at least in the Dragonlance Adventures incarnation).
I did say that it needs a baseline
. If you happened to quote a bit more of that paragraph you would have included the part where I said that elves, drow and orcs are all the baseline creatures with baseline alignments. You can THEN vary from that to create something new and those baseline creatures can also be tossed as needed. The same applies to planes, so when you encounter a world that doesn't deal with those planes it is perfectly fine to toss them, but for most games they are going to be present in some form.
I even consider 4e to be one of those forms, as they are very similar in a lot of ways. My comments about DnD had the great wheel for a while still applies regardless of when the wheel was first introduces. To a lot of people DnD isn't DnD without a lot of little details. The more of those you toss or replace for little or no reason the more you start to stray from what people consider the core or baseline of DnD. That is fine to do from time to time, new races are cool. But rarely does everyone get on board about changing everything all at once. Recognizing this is a step in remedying it. It doesn't mean they have to think all the changes were bad but it does mean they have to realize they weren't all universally accepted either. It doesn't mean that they are trying to anger 4e fans but it does mean that several changes which 4e fans herald aren't widely accepted outside of the 4e fanbase - including this new cosmology.
Yes. I never really understood the rationale for this. (I'm Anglo/Irish-Australian, so some of that slang is part of my childhood vocabulary. What's it doing in Sigil?)
The only thing I can think of is some people want a pseudo-industrial citystate to play around in. It is kind of like the Legend of Korra setting, where it is clearly a mishmash of a lot of different cultures in the 1920-1930s most notably New York and Hong Kong (if I remember the press releases correctly). The new vocab might be part of that. A couple posts back someone mentioned it sounding like the Bronx, that is an element of it I think.
I could be equally off-base, having never actually encounter Sigil (or traditional planescape) in game.