TBut the nine-alignment system has been a long-time aspect of D&D lore AND mechanics.
That's really nowhere near the same thing as being good. There are a lot of other game concepts that were introduced between OD&D and AD&D that were
just plain terrible, inexcusably bad, that were thankfully removed much earlier than racial ASIs and the alignment rules. Exceptional Strength. Separate Strength maximums for female characters. The weapons vs. armor tables. Training rules.
The Lawful <-> Balance <-> Chaos alignment system of Moorcock is
weirdly specific compared to non-Moorcockian fantasy but at least it's a consistent and coherent system.
Adding the Good <-> Evil axis forces it to
break down entirely, especially when you try to retain the concept and value of Balance
between Good and Evil, which some AD&D (and later) material tries to flirt with. Then there's the fact that almost all AD&D (and later) material
directly states that Lawful Good and Chaotic Good are
equally good, then
heavily implies that Lawful Good is the "greater good" of Good alignments, and then
in practice in most D&D settings established that Lawful Good is an
oxymoron.
No, it's not
hyperbole. The rules and mechanics surrounding the nine-axis alignment system in AD&D (and later) have always been a treacherous mess because they're built on a foundation of
absolute goddamned nonsense.
If we're going to start resurrecting incoherent jank from last century's D&D, why can't we start with multiclassing or something? Those rules were bad, but at least they did what they were supposed to do and they're better than we have now.
Over time, the mechanics related to alignment have been watered down (e.g. no more alignment languages) but 5e still nods to those of us who continue to use alignment by having some magic items usable only by characters of certain alignments.
You don't need the existing alignment rules for this. You especially don't need someone to
bring them back for this.
Does the item have an ethos? Does the player fit that ethos?
Done. Simply having that conversation with the player, on a case by case basis is simpler, easier, and
more fair than trying to lump every sentient being in your world into that ridiculous 3x3 grid.
In my current campaign, alignment is important. It is less about a character's psychology and character (although it can affect both) and more about a cosmic battle over how reality should be ordered. Yes, the great-wheel and similar cosmologies aligned to alignment are ham-fisted, but this is a game, not an exercise in philosophy.
So... to be clear, the reason that everything in the game world needs to be assigned an arbitrary spiritual/ideological category pertaining to the metaphysical underpinnings of the cosmology... is because you don't want your game to be bogged down in a bunch of philosophical minutia? Is that really an accurate summary of the position you're taking here?
At some point, if you keep taking away these weird bits of D&D, it stops becoming D&D. Already alignment is clearly an optional subsystem in 5e. I don't see needing to get rid of it entirely. You can have settings that don't use alignment and those where it features front and center.
Sure. I agree with this. But that means that the alignment rules need to be marked (optional) the way feats and multiclassing are, and the list of D&D settings that use those rules wouldn't be "all of them". They shouldn't even be in the core rulebooks, because if multiple settings are going to be based on such a weird and specific and frankly boring and awful concept, they should at least
do it differently.