Where exactly inside the "Great Wheel" is Eberron in your opinion? Is the suggestion here that the entirety of the Eberron cosmology is within the Prime Material plane? You have the planet Eberron, its sky Siberys, and the thirteen orbiting "planes" of Eberron... all within the "Eberron bubble" within the Prime? So that Eberron's "planes" are not actual planes at all (at least not on the same level as the the elemental planes, the Astral plane, Mount Celestia, the Abyss, Ysgard, Carceri, etc. etc.?) that the Greyhawkian sages imagine the cosmology to be?
That seems... silly, to me.
So Fernia in the Eberron setting... which is essentially the Elemental Plane of Fire for Eberron... isn't actually a part of the Elemental Plane of Fire, but instead just a pocket of hot gas within the Prime Material plane that "sages" from other Prime worlds say aren't the same? Who gives them the authority to make that declaration? That their opinion on how the "inner" and "outer" planes are set up is the "true" way the universe/multiverse is set up? I mean it seems to me that Fernia is just as much the same "plane" as the Elemental "Plane" of Fire... it's just a metaphysical space of intense heat. They are one and the same from a conceptual point of view of the people in these worlds who are observing them.
It's just that the one thing they are not is just ONE location. You can't use the Elemental Plane of Fire / Fernia or whatever a sage might call that metaphysical space to go to other worlds. You can't enter the Elemental Plane of Fire from a portal in Faerun... walk a couple miles in the plane, that plane turns itself into "Fernia", and then walk out through a Manifest Zone and enter Khorvaire. That's not how any of this works. Especially considering how any of this "works" is mutable and changes all the time.
Now... if that isn't the suggestive argument, and the actual argument is that Jeremy Crawford and the WotC staff are declaring from a meta level how all of this multiverse stuff "works" and thus all of our worlds are beholden to their declarations... that isn't true either. Jeremy et. al. are unreliable narrators about ALL of this.
How do we know? Because they are re-writing things that have come before. And what they are writing is no more "correct" or "real" about the D&D Multiverse that what had been written by people for editions past... because in 10 years time when a new staff comes in to write 6E... there's a pretty good chance that those people are going to "re-write" what Jeremy and the 5E staff have "declared" about the truth of the D&D Multiverse. Heck... the 6E staff might very well REMOVE all references to the D&D Multiverse for all we know. Which means it is silly to think that Jeremy has changed anything "real" in D&D. If what he writes can also be changed... then nothing is "real". Nothing is "true". He can write down whatever he wants... and while many players might go along with it and turn their pocket of the D&D Multiverse to match what Jeremy says... there are going to be a crap-ton of people who don't.
And this leads into another point about the whole "D&D Multiverse" thing that I don't think Jeremy et. al. really delve into (or just ignore), and thus most players don't tend to think about. The whole point of calling this a "Multiverse" is to get across the idea that there IS NO ONE set thing. Despite claims to the contrary... there is not ONE Tiamat. There just isn't. There is one Tiamat for every player of D&D past, present and future. And in fact, actually MANY Tiamats for those players past, present, and future.
How do I know? If my players played the Tyranny of Dragons adventure and KILLED Tiamat... Tiamat doesn't suddenly disappear from the entirety of the D&D game. My game in my pocket of the "D&D Multiverse" has absolutely no effect on anything else. If in my game the Astral plane just ceased to exist for some reason... that also would have no effect on the D&D Multiverse. Likewise... even if Jeremy et. al. wrote in an upcoming book "Demogorgon has canonically been eradicated from existence and is no longer a demon lord in D&D"... that has NO EFFECT on anyone's else's pocket of the D&D Multiverse. Because I could the very next day run an adventure and have Demogorgon show up. Heck... I could have SIX copies of Demogorgon show up. Because that all happens in MY PART of the D&D Multiverse. And what I do in my game does not impact anyone else's game in any way, shape, or form. Just like what is written down in any of the books does not impact anyone else's game in any way, shape, or form. That's entirely why we call it a Multiverse in the first place.
So for anyone to get mad that Jeremy et. al. have written down supposed "truths" about the Great Wheel or any cosmology in a book is just as much a waste of time as it is for all the players who are getting bent out of shape that Jeremy et. al. are starting to not put default alignments in monster statblocks anymore. What are written in these books do not matter, and do not impact your game. All they do is change how other players might now look at their own game... which will increase the number of games looking that way. And thus YOUR way of looking at the game probably becomes less popular. But you know what? Who cares? It doesn't matter how many people share your worldview on how D&D "works", because none of those millions of other people play in your game.
"No one is taking your books away" is a common refrain we hear regarding this kind of stuff. But what it really should be is "No one is changing your corner of the D&D Multiverse". You can make your corner however the heck you want, and NOTHING anyone else ever says, declares, re-writes, invents etc. etc. etc. in the past or in the future will ever change that.