Well, I see your perspective. I too feel some dissonance about focussing on snarky "gray" Sigil as *the* center and epitome of the D&D Multiverse. Since all Outer Planes, including Sigil and the Outlands, are conceptual entities, the D&D Multiverse could be presented as "pluri-centric", where any place in the Multiverse could be viewed to be the legitimate center. Granted, the Concordant Opposition/Outlands/Sigil-centered model was the default cosmology in 1E (Great Rectangle) and 2E (Great Wheel). Crawford explicitly denigrates (or at least pulls back from) the 3E pluri-centric model. Yes, it too annoys me when the Sigil scholars are presented as the only ones "in the know" and that the other worlds (such as the people of Krynn) are bumpkins. Such a presentation actually conflicts with the assertion that the Outer Planes are conceptually fluid. And also conflicts with the several alternate cosmologies offered in the 3E era (such as in the 3E MotP), which are also "official" depictions of the D&D Multiverse. If I were in charge of D&D's development, even while firming up the interconnectedness of the D&D Multiverse, I'd make sure that the "pluri-centric" visions of the multiverse--the Great Tree of Forgotten Realms), the Five Spheres of Mystara, the Orrery of Eberron, and so forth--were fully addressed by 5E cosmology.I think you may have misinterpretted what I was getting at, maybe if I break some of my thoughts down further it will help.
In response to your thoughts on my campaign world, you are correct, I don't care if the worlds are connected. One way or the other, does not matter to my games. I've already got seven or more planes of existence to juggle, doubling or tripling them by connecting whole other worlds does not appeal to me.
However, I think a larger part of the discussion and my personal thoughts goes to the second point. I don't care how they are connected. And that is the entire premise of settings like Planescape and Spelljammer. They seek to explain exactly how (From your list of examples) Murlynd visited Earth. They give reliable ways for any person who has the knowledge to go from Plane of Existence A to Plane of Existence B. And that's great if that is what you are into, but I find it completely unnecessary. If I wanted throw Superman, Legacy (Paulina Parsons), and Atlas into Waterdeep to fight The Cult of the Dragon... I'd just do it. I don't need to explain how those points all intersect. And it creates a series of problems when you try and spell it out.
Take Star Trek as an example, the people on a given planet may have a certain perception of reality, such as space is full of ghosts and the stars are the gods who watch over them. However, they are fundamentally wrong about that reality, and the reasons the crew of the Enterprise doesn't come down and correct them is because doing so violates the right of those civilizations to learn about the truth for themselves. However, it breeds a sense of superiority doesn't it? Anyone visiting that planet from space knows that these people have no idea what is actually going on, and therefore they are somewhat lessened in comparison.
We have that already. We have the idea that planescape scholars scoff at people stuck in one plane, because they don't know the "real story". And we can scoff at them, because the planescape people don't realize they are all characters in our made up stories to entertain ourselves (multiverse theories being what they are, all multiverses are connected with all other multiverses, and an infinite array of potential truths).
But we don't need any of it. We don't need to explain how it is all connected and what is all looks like when you zoom all the way out. We can just as easily (as @ccs points out) just use the planeshift spell, or a macguffin, and get the result of going from point A to point B.
That's the bigger thing for me personally, these settings answer questions I don't need answers to. Other people want them and other people find the neccessary to their enjoyment of the game. Great, go nuts, but I don't need flying ships through explosive magic stuff between galaxy sized crystals that are grains of sand to explain how some guy one time went from Oerth to Earth and shook hands with Stan Lee. The characters aren't going to meet Stan Lee in my game, because it would detract from the story I am telling at the time. And someone telling me that despite my dislike they are already connected anyways.... doesn't matter. I don't care how long they've been connected, how many times they've been connected, or how important it is they are connected. It is never going to come up, my players aren't interested.
And I likewise feel some dissonance about how other D&D concepts of outer space seem to be fully discarded in 5E, as they were in 2E - in favor of the Crystal Sphere space cosmology. In 1E, each world had it's own *universe"! Each world - Toril, Krynn, and Oerth - was an entire distinct Alternate Prime Material Plane, with its own galaxy and universe, which could not be reached via space travel, no matter how far you flew - because they existed in alternate material planes. And in BECMI D&D, outer space had galaxies, not crystal spheres - there is even a Star Trek-tech sci fi culture which rules much of the galaxy - the Galactic Federation. And yet another outer space cosmology was offered when the Immortal Set was totally revised and replaced by the Wrath of the Immortals boxed set. And what about 4E's (Nerath's) outer space cosmology where "outer space" was the Astral Sea, which could be reached via astral skiffs?
Is Crawford just shoehorning this all into a 2E cosmology? I'm all for firming up the interconnectedness, and for acknowledging and building upon the dozens of Spelljammer products. But the various other official D&D conceptions of outer space also need to be acknowledged and retconned into the 5E cosmology.
I'd do something bold like this: make the Phlogiston a "parallel plane" (similar to the Feywild or Shadowfell) that can only be reached via particular kind of magic (spelljammer magic). When on a spelljammer ship, you've shifted into a quasi-material state, and you experience the world differently.
Or you can travel in the astral plane (the Astral Sea) in an astral skiff or astral spell.
Or...you can travel using fully material means, such as a rocket engine or airship (using non-spelljammer magic). In which case, when you pass beyond the edge of the solar system ("crystal sphere"), you're in "real-world"-style outer space.
And the "crystal sphere" is experienced differently depending on what kind of vessel you're approaching it. When in the Spelljammer plane, the sphere appears as described in the Spelljammer books - a big hard shell where the stars are merely radiant dots and such. When in the astral plane, the crystal sphere is experienced differently. When in fully material mode, no Crystal Sphere at all is seen or experienced - just the cold black borderless edge of the solar system.
Outside of the solar systems: when you're in the Spelljammer plane, you enter a shared cosmos where the D&D worlds can be reached as described in Spelljammer. When you're in a material ship (rocketship, airship), you'll never find the other D&D worlds, because they actually exist in Alternate Prime Material planes.
When travelling materially, you just keep travelling along in a material universe which is not shared by any other main D&D worlds, as was implied in the 1e MotP. Each has their own entire universe with a very distinct cosmology. For example, in the Krynn Universe, the stars really are shaped like that in the sky (they're not just glowing dots on the wall of the crystal sphere). And they really did disappear. However, when shifted into the "Spelljammer Plane" in Krynn, the events were perceived as described by Spelljammer.
As for BECMI - officially the BECMI Multiverse exists in an entirely different Reality than other editions. This "Reality" concept could also be used to explain the difference between 5E Multiverse and other editions.