[MENTION=2067]Kamikaze Midget[/MENTION] : I'm trying to understand (from a practical viewpoint) your stance here... are you arguing that there shouldn't be a baseline?
There's a few different ways this could look from a practical WotC standpoint, but I might prefer a building-block approach:
For the planes. The DMG mentions the planes in the context of introducing what planes are, why a DM might want to use one, and how you might want to use them in your games (possibly in a Worldbuilding chapter). Then it provides a few planes that you can use out-of-the-box. The Abyss. The Nine Hells. The Feywild. It doesn't talk about how these go together so much (it maybe reference the Great Wheel as one model, and has a picture). You can probably do that in 3-5 pages, depending on art and whitespace and level of detail.
For alignments. The DMG mentions alignments by talking about how they might be used, what they're good for, what they're not so good for (this maybe happens in a Characters or NPC's chapter). It gives you a few alignment systems out of the box. Law-Neutral-Chaos, Nine Alignments, LG-CE axis. You could probably get away with 2 pages on this.
Then you have the MM. The Intro (or chapter 1 or maybe the Monsters chapter in the DMG or whatever) lays out what monsters are, what they're used for, how to make one. How to customize them by filing off the serial numbers and descriptions. The list of monsters is a list of examples you can use out of the box, like alignment systems or planes. When Demons come up in the MM's alphabetical list, it says something like "The Demons of the Abyss are creatures of chaos and evil incarnate, destructive monstrosities that strive to tear apart the world, obliterating it in an bacchanalia of destruction and anarchy. The Abyss that they live on is thankfully isolated from the mortal world, but wicked priests and debauched cultists practice dark rituals that bring these creatures from beyond into roiling, fleshy being in the crypts, dungeons, and dark temples that they inhabit."
And then you have the Marilith and the Hezoru and the Vrock and the Balor.
And then maybe you also have a sub-heading: "Elemental Demons," and this says that "In the Nentir Vale setting, the Abyss roils at the center of the Elemental Chaos, and this elemental nature infuses the demons that come from it." It then maybe has a template that you can add to the above four demons to give them a more "elemental" cast (Marilith: Earth! Vrock: Wind! Hezoru: Water! Balor: Fire! Galbrezu: Heart? Go Planet?) that maybe changes their type and gives them an elemental attack.
And, hell, if you have the page count (and if you think a variety of demons is an important goal), maybe you have a sub-heading for "Demons of the Blood War" that says something like "In the Planescape setting, demons have an ongoing war with the devils (q.v.), and the demons' edge in this war is their endless chaos and variety." Then we've got a template that you can add that gives them an alignment subtype and lets them smite law or have chaotic fecundity or something.
Someone who doesn't want to bother building with the blocks has this slot-a-in-tab-c kind of match. Okay, there's the Demons, and there's the Abyss, and there's Chaotic Evil, and these all work together. Sounds good. And ooh, I like the sound of that Blood War stuff, cool, now I've put together a whole
thing.
Someone who wants to play D&D without demons or the abyss or Chaotic Evil (or who wants to separate out those things) doesn't see any suggestion that they're not doing D&D right if they exclude them. Maybe we've got a DM going with a vaguely Christian Mythos kind of world and he just crams demons and devils together as different shades of the same monolithic evil, doesn't use the Blood War, doesn't use the Abyss, but otherwise leaves them basically unchanged. A 5e written like that makes it easy, because it doesn't say "All D&D Demons are from the Abyss and they are like this," it says "If you want to use demons from the abyss in your D&D, they might be like this."
This really shows the magic when dealing with, say, the Eladrin. You can have an entry like "Eladrin of the Feywild" that is basically 4e Eladrin, and "Eladrin of Arborea" that is basically 3e Eladrin and a DM can use one or both or neither. And if none of the core sources mentioned the Feywild or Arborea? Well, those gaps are for DMs to fill in...or they can wait for the Manual of the Planes...or whatever.