Disclaimer 2: I am not interested in arguing or being "right." More in observing the historical changes and seeing what they might mean. I'm not trying to convert anyone to running their own games in any particular way. Talking the history of the multiverse is fun, arguing about what is better is not (for me, you're welcome to whatever you want to say!).
Ok, actually read the full post! Full disclosure, I'm ignoring Disclaimer 2. I know you're not trying to convert anyone, but after reading what you think about "seeing what they might mean," I kind of disagree with your leaps so I'm going to point them out. I'm not going to say what is better or not, but I do disagree with what you seem to think the design changes actually mean.
Firstly, I don't know how the Manual of the Planes stops players from traveling from world to world. It's clearly not true, as you have a bunch of characters cross-pollinating within both Spelljammer and Ravenloft from various settings.
A. Every world must be a D&D-type fantasy world. While there are allowances for space hippos, there is no allowance to spelljam to Gamma World.
B. There can be no variations in rules. Prior to 2e, many gamers were quite used to going back and forth between 1e and BECMI (at the very least, they would play the modules interchangeably). You could not, however, travel to the the well-known world of Mystara.
Is there an actual source for these assumptions? I haven't seen anything that states "D&D are fantasy only, you can't sci-fi!" If not this is a big assumption.
Two years later, Dark Sun (Athas) revealed that this change in direction was permanent. Athas did not want your cross-pollination with other worlds. "No creatures from the SPELLAMMER Monstrous Compendiums live on Athas." Athas is treated as cut off, separate, away from the deities that populate other planes. It is treated as cut off. It is remarkable, yet unsurprising, that the very first edition post-Spelljammer both declines to use Spelljammer and also adopts a cribbed cosmology; this is something that would repeat time and again moving forward.
To be clear, the whole premise of Dark Sun (that it is a post-apocalyptic wasteland) kind of breaks down if people can just up-and-leave. It's also not even
true; there are actual Dark Sun modules where the Githyanki invade Athas, showing that Dark Sun isn't as cut-off as people first assume.
I'll be honest here, I am so very very confused by this whole series of posts. The changes you're saying happened did indeed happen, but they haven't invalidated any aspect of Gygaxian portals and multiversal concepts. It just adds some structure to explain
how those concepts are possible.
5E goes out of its way to embrace several of the Gygaxian elements you love. Dungeon of the Made Mage pulls elements from other settings into its nutty expanse of material, including Spelljammer. Characters like Acererak from Greyhawk and even Critical Role characters have appeared in Forgotten Realms content. They even released the Lost Laboratory of Kwalish which is a direct call-out to Expedition of the Barrier Peaks.
I love the amount of research you're doing and also love the nostalgia of TSR (which I share), but I do disagree that the material of today somehow invalidates it... it's all still very possible, and still being sprinkled throughout new material.