I'm a big fan of the concept as in 2e Planescape, but not so much when you force settings into a different cosmology different to the one they had previously or for multiple editions already (see 4e's World Axis planes and its content rather aggressively pushed in various form into settings other than Nentir Vale).
I don't think it is that simple. You blow a big hole in the Tiamat example by pointing out that Krynn, the (Krynnish) Abyss, and Takhisis were never supposed to be part of the Great Wheel, and you're right to do so, but there are plenty of examples of core D&D mythology that got passed over in the creation of Planescape. My favorite example (and one of the oldest) is the Wind Dukes of Aaqa.
The Wind Dukes of Aaqa are original D&D stuff, first appearing in the 1976 Eldritch Wizardry booklet, in the description of the Rod of Seven Parts. Planescape launched in 1994, but the Wind Dukes of Aaqa don't enter the new setting for two years, with the release of the 1996 Rod of Seven Parts boxed set. When they do show up they have been well and truly shoehorned into the Planescape lore. Accommodating the Queen of Chaos and Miska the Wolf-Spider into what has been created for the tanar'ri requires even more force, and the result feels like a bad mashup.
Planescape, like the World Axis, was largely created out of whole cloth for the 1994 release. It drew on existing material in broad strokes, but Planescape ignored at least as much detail in pre-existing material as it adopted. Whole planes changed names, even geography. The World Axis was more destructive, absolutely, but it was more destructive simply because in 2008 there was a lot more to destroy than there was in 1994 -- again, because Planescape invented most of it between 1994 and 2000.
The reason why I have a soft spot for the World Axis is because it tries to be a part of D&D. It acknowledges the fact that a D&D world's cosmology would impact the D&D world, and it is designed accordingly. The Wind Dukes of Aaqa are in there, as is Tharizdun, the Elder Elemental Eye, the Abyss, Mount Celestia, and a host of other classic D&D cosmological ideas, and they are presented in a way that is relevant to playing D&D in the Nentir Vale.
And that's really why the World Axis comes out slightly ahead these days when I weigh it against Planescape. Planescape is magnificent, but it just isn't relevant to playing D&D in the worlds it is supposed to support. In fact, one of the core conceits of the setting is the ignorance of "Primes" to the way things operate on the planes.
But how can the factions be /so important/ on the planes but have virtually no impact on Toril or Oerth, whose inhabitants supposedly 'believed' the outer planes into existence in the first place? Why is the Prime Material completely unaffected by the inconceivably vast conflict of the Blood War? Why are the realms of the gods /footnotes/ in Planescape, when they are the most important aspect of the planes to any prime material setting?
TL;DR:
Any "core cosmology" in D&D5 has to logically support the worlds it contains, and the fact is that Planescape just wasn't written that way. It was written to be its own strange bird -- and that's great -- but it's not going to be of any use in any "core" sense, unless all the D&D settings, old and new, are adapted to incorporate its strange birdness.
The World Axis isn't much better, to be fair. The elimination of the Great Wheel and Elemental Planes, among other aspects of classic AD&D cosmology, was just too much. What Mearls has said about the new cosmology sounds like the design team understands this. I hope they do.
And of course, if you don't want a shared cosmology, don't use it. But D&D has always had one to some degree, and the flaws of previous drafts don't invalidate the potential of future drafts.
