My general preference for alignment is thus:
Natural creatures like humanoids or giants don't have fixed alignments, but the cultures they come from may nudge them in certain direction. If the orc tribe Krusk grows up in are raiders and pillagers, Krusk is more inclined to be CE, but that's not a given nor is it reflective or all orcs.
Unnatural creatures (aberrations, fey, undead, and maybe dragons?) have unnatural moral compasses and thus are typically found in a small band of alignments, though exceptions exist. You can assume a specter is most likely evil, a satyr is mostly likely chaotic, and a red dragon is most likely evil, but it's not 100% guaranteed. These creatures simply don't see morality the same way a human or elf does and it's part of what makes them UNnatural.
Extraplanar entities (Celestials, Fiends, Elementals) are created and molded by their alignments; they are the exemplars of these views. Other morality isn't just unseeable to them, it's an anathema. Few of these creatures ever change their outlook enough to change alignment, and those who do (Zariel, Grazz't) are legendary for it. You should be able to count the exceptions to them on one hand.
But that's just me, I like having some things be stronger aligned than others. I kinda like the idea some things are just so alien that they don't see the world like a 21st century human does.
My general preference for alignment is thus:
Natural creatures like humanoids or giants don't have fixed alignments, but the cultures they come from may nudge them in certain direction. If the orc tribe Krusk grows up in are raiders and pillagers, Krusk is more inclined to be CE, but that's not a given nor is it reflective or all orcs.
Unnatural creatures (aberrations, fey, undead, and maybe dragons?) have unnatural moral compasses and thus are typically found in a small band of alignments, though exceptions exist. You can assume a specter is most likely evil, a satyr is mostly likely chaotic, and a red dragon is most likely evil, but it's not 100% guaranteed. These creatures simply don't see morality the same way a human or elf does and it's part of what makes them UNnatural.
Extraplanar entities (Celestials, Fiends, Elementals) are created and molded by their alignments; they are the exemplars of these views. Other morality isn't just unseeable to them, it's an anathema. Few of these creatures ever change their outlook enough to change alignment, and those who do (Zariel, Grazz't) are legendary for it. You should be able to count the exceptions to them on one hand.
But that's just me, I like having some things be stronger aligned than others. I kinda like the idea some things are just so alien that they don't see the world like a 21st century human does.
I should preface this by saying, I don't really play D&D that much anymore (mostly I play other systems and my own systems), and my default preference is actually for more no alignment or morally gray worlds (or systems that cleave better to more real world conceptions). But I do like alignment in D&D (I think because when I do return to D&D, it is because I want those classic D&D trappings, and I think there is something about D&D that just works and is highly functional----in the sense that I can always reliable start and continue an ongoing campaign using D&D, whereas some other systems, even ones I like, may not be as consistently reliable....not quite sure if I am expressing this well). And with D&D alignment in particular, I think one of its challenges is, because it has changed much over the years, it has to support a wide variety of approaches to the game to satisfy different expectations.
I think another expectation is alignment in service to this notion of a cosmic battle where different groups have taken sides. I don't think this needs to mean that a big C chaotic orc, or a CE orc has to always be so (unless you are dealing with orcs who are literally manifestations of evil), but that as a general rule the Orcs are aligned with Chaos or Evil, or Chaotic or Evil forces. This makes sense in a cosmic struggle setting, but it makes less sense in a more grounded setting.
Another expectation is simply as shorthand to make things easier to run. So you might have monsters and races with a wide variety of cultures, but the alignments just denote tendencies, and are largely a tool for the GM to use for handling encounters easily.
Now my favorite setting was always Ravenloft. Which didn't really have an abundance of orcs, and most monsters were in service to horror, so alignment there mattered in terms of dealing with the morality of the setting (because of things like powers checks) and with managing NPC behavior and knowing the flavor of evil a given monster represented. I found it handy, but in truth Ravenloft was always more concerned with the divide between good and evil, than with questions of law, chaos and neutrality.
And I think there has long been a history in the game of people using alignment how they want to, ignoring it, etc. I know there are many campaigns I ran where alignment wasn't really treated the way it is in the rulebook.
I guess what I am getting at, is I can see the challenge alignment presents to designers for dungeons and dragons because it needs to be able to handle a baseline system that can be used for any GM to make their own setting, allow for the GM to take any number of the above and other approaches, and be flexible yet not different enough to handle a range of official settings. That is a tall order, but I think serving that order, and the wide ranging player base, is one of the keys to D&D's success.
This does raise the question though: if you are unsatisfied with elements of D&D (not you Remathilis, just people in general), and I certainly found I was unsatisfied with WOTC D&D by the late 2000s; why not try other games and systems? There are games with no alignment, with more nuanced setting cultures, etc. And those games definitely could use more fans. I am a bit proponent of playing other types of games because the period I cut my teeth gaming was before the d20 boom, and it was just expected every game had its own system, and every group would play more than one type of game. D&D was our bread and butter, but it was pretty easy for me to pull away from D&D and make that less central to my play experience when I realized what I wanted from the game, wasn't what the makers felt they should be making. I get that D&D is kind of an important focal point in the hobby. But it really did eliminate a lot of negativity in my thinking about RPGs to go for other games.
Regarding your remarks on extrapljnar creatures (which I think are accurate), one thing I always found interesting about those types of beings, is in D&D they are created evil, created good, etc. And I think that is fine. But I always thought, in a cosmic battle campaign, it might actually make more sense to draw on real life lore about angels and demons. Angels and Demons, at least in a lot of traditions, are both the same kinds of beings: beings of pure spirit, who have more perfect knowledge than humans. The only difference is they used that knowledge to make different choices in the cosmic struggle of good and evil very early in creation. So the idea is angels chose to be good, demons chose to be evil, and this choice was a permanent one because they had full understanding of what that choice meant (whereas humans don't have the level of knowledge to make such an informed and permanent choice). I may be getting bits a little wrong here, no expert, but that is the gist. I always thought that approach fits nicely into a world modeled after things like Three Hearts and Three Lions (and similar novels) where there is a cosmic battle where groups are picking sides (and humans maybe have more openness on the choice, because they are the stand in for us making a choice when confronted by these forces)
