That all seems to come down to having thematic elements in your game. The struggles of the forces of Law and Chaos, God and Evil can play out in an alignment-less game and imo they become all the more interesting because there is no scoreboard for how any individual is doing in those struggles.
In your examples, it's not that alignment is "real", but that moral choices have physical consequences. IMO my mechanics work even better than traditional alignment for those sorts of overt struggles. They handle the transformation of Melkior to Morgoth as well as Elric's journey. It has been more than 20 years since i read Three Hearts and Three Lions, so forgive me if i've got it wrong, but Holger is an utter outsider to the world and his observations of how things work should not be taken for factual cosmology, but even if you do want that cosmology, it still seems like my system works better. Law and Chaos exist and creatures are created by it, shaped by it, but not limited to their source. From the quote you provided: "humans were the chief agents on earth of Law, though most of them were so only unconsciously and some, witches and warlocks and evildoers, had sold out to Chaos."
Apparently i have switched to full-on advocacy mode. I probably should have seen that coming.
To criticize it from another angle, alignment reduces player agency. If they are doing something because their alignment demands it instead of because they want to or because it's fun, then they have less immediate control of their characters. Sure, they chose the alignment in the first place, but what seemed fine when designing the character might not fit after a few levels in the world. The obvious answer to that is (unless they're a paladin) they have varying degrees of freedom to change their alignment, and if, mechanically speaking, changing alignment doesn't matter all that much for most characters, then why use it at all? For situations where a particular set of behaviors is encouraged or required, more concrete forces can easily fill the void that alignment leaves. Culture, Religion, even the direct action of the Gods and Goddesses themselves.
To be clear: i'm not suggesting that any character should do anything the player feels like doing at any time. Choices should be based on the character's concept, history, the situation at hand, and the player's wishes. No meta-game labels are necessary.
In the end, i'd say the proof is in the pudding. It's easy to hang on to alignment because it has been in our games for 30+ years and we're all familiar with it, but because it's such a weak mechanic, it's also easy to drop it entirely. Try it next time you play. I doubt you'll miss it.