It's a return to a common cosmology as in 1e and 2e
Before Planescape was a campaign setting there was a default D&D cosmology in 1e, with a presumption of connection to each campaign setting
I don't think this is true at all. If you read Gygax's discussion in his DMG of travelling to other planes, to the moon (on the back of a pegasus!), to the worlds of Boot Hill and Gamma World, I think there is no assumption of a default. The assumption is that the GM will develop his/her world as seems appropriate. The placement of the planar scheme in an appendix to the PHB reinforces that - like bards and psionics it is optional, not a default assumption of the game (other perhaps than the astral and ethereal planes, which also figure in magic such as Astral Spell and Potions/Armour of Etherealness).
Some Monsters in the Monster Manual have planar locations specified which figure in the PHB scheme (eg there are references to the Hells, Gehenna etc) but in many cases this is the barest of flavour text (eg Demondands could be from anywhere), and in some cases, like Bahamut living "beyond the east wind" it is a flavour text that departs from that of the PHB appendix.
Manual of the Planes perhaps marks the turning point - Jeff Grubb, with his parable of Fibber McGee's closet (I only know the name of the character from Grubb's intro, and am trying to get it right from memory here), explains that his goal is to present everything planar that's been done in D&D as part of a coherent whole. One notable disply of this is his treatment of all the pantheons in DDG as co-existing, whereas DDG itself seems to assume that the GM will choose which pantheon or pantheons might exist in any given gameworld.
But MoP is 1987, which is a late product as far as 1st ed AD&D is concerned. If you look at Dragonlance, for instance, which predates it, the cosmology, including the notion of "The Abyss", don't correspond with the PHB schema at all.
I think that the original presentation of AD&D took it for granted that the GM would decide what cosmology to use, what parts of the PHB appendix might be included, which pantheons were "real" in that campaign world, etc.
2nd ed AD&D does seem to have begun from the get-go with a MoP-type orientation.