I think we've got two different things going on here though:
In Nerath, the World Axis is literally the arrangement of the planes. Devils are Immortals from a specific Plane(t) within the Astral Sea, the Nine Hells; Demons are Elementals from the Abyss within the churning center of the Elemental Chaos. The concept of a united Fiendish category from the "Lower Planes" that is contrasted with Celestials from the "Upper Planes" can't exist in Nerath because there are magical effects that target Immortals and vice versa for Elementals, and they affect Devils and Demons differently. Devils are hit by effects that would also hit Angels and Devas (5e term: Aasimar) and whatnot. Demons are hit by effects that would also hit Djinn and Archons (5e term: Elemental Mymidons) and whatnot. That's not reconcilable with the Great Wheel.
Someone in Nerath could model the cosmos as a Great Wheel and loop devils and demons together and treat the Elemental Chaos as 4 distinct, pure, elemental planes, but they'd be wrong, and provably so.
But in the (3e & 5e) Forgotten Realms, the planes could be modeled as a World Tree or as the Great Wheel, and both are equally valid models. In 3e Faerûn, the World Tree was presented as an alternate configuration of the planes that were in the 2e Great Wheel. This model had the benefit of coalescing with the Nerathi planar model in 4e, when the mess of the Spellplague and Returned Abeir coincided with planar collapse of the World Tree - the Inner Planes falling into one another to form an Elemental Chaos, and many of the Outer Planes drifting into the Astral to become akin to Nerath's Astral Sea. We see here an (out-of-universe) mirror to Nerath's planar set-up, no doubt designed by WotC so that they wouldn't have to spend time on how the Forgotten Realms have a different planar set up than the assumed setting of the 4e Core Rules. But in-universe, we have two separate models (Great Tree and Great Wheel) that were able to describe the cosmology pre-Spellplague and post-Second Sundering, and the World Axis model that more accurately describes the planes during the Spellplague years. The former two are metaphorical representations of the same infinite planar spaces that can be debated on which is a better representation a la Quantum Mechanics vs General Relativity. The latter is a literally and cosmically different set-up.
So we've got worlds that for all of their known existence (Eberron, Nerath, MtG Planes) and/or a period of time (Forgotten Realms) have a radically different cosmological set-up. And then we've got planes that have their own cosmological set up but it's really just dressing that describes the same Great Wheel in a different way (Greyhawk, Abeir-Toril, etc).
If we want to cram Eberron and Nerath etc into the Great Wheel, that's possible, but it loses much of what makes those settings unique and a lot of assumptions have to change in the process. Maybe Eberron's "planes" are actually planar gateways to Great Wheel planes. That's do-able, I guess. Maybe Devils are pretending to be Immortals to the citizens of Nerath, and maybe Obyrith trickery has something to do with the Elemental stuff for Demons in that setting? She's not called the Queen of Chaos for nothin'. But in both cases, the worlds work better as their own pocket realities, much akin to how Theros has it's own three planes and has Gods and Nyxborn that live in Nyx, akin to the Upper Planes, Astral Plane, Plane of Dreams, and Feywild all tied into one, while the Underworld is akin to the Underdark and the Lower Planes and the Shadowfell all tied into one, where devils are called demons and all fiends are underworld beings of the same class.
I'd much rather consider that each D&D Plane, MtG or otherwise, has it's own cosmology that surrounds, penetrates, and sustains it. But those realities are not mutually exclusive, and some beings can move between them. Heck, the Far Realm essentially is this concept - it's a plane that's not really a plane but rather an alternate reality invading our cosmology. The world of the Obyriths is this too, as are the Phyrexians in Magic.