I think you're being a bit extreme yourself. There's nothing supremacist saying that Europe in middle ages was pretty much all white. If the writer had in mind that period, and wanted to tell the story in that spirit and theme, and also based on western and northern mythology, then you normally would expect them to be all white. I don't know about you but I'd be extremely surprised to see whites or blacks in one of Chinese dynasty mythological movies with flying swords and stuff.
There's an important distinction to make here. You gave the example of Chinese mythology - those myths (like any) are based on stories that people believed to be real, and are set in the real world (where yes, almost everyone would be Chinese). But LOTR is a work of fiction, nobody believes it is real, and more importantly, it is not set in the real world. A Chinese myth would be based in China, a Norse myth would be based in Scandinavia. Where is LOTR based? So to argue that 'everyone would be white' is meaningless, as this is a world that never existed. It's like saying, 'you wouldn't get
that type of alien living on
that planet' in Star Wars. There aren't the same 'rules' that would exist with something set in the real world, even if it's fictional.
I've seen some people saying it's like putting white people in Black Panther. But the difference is, while BP is set in a fictional country, it's still set in the real world, where in an isolated African country almost everyone would be black. And also, there is no way they would react the same way if white people
were cast in BP, which just shows their hypocrisy.
There's nothing in LOTR that says there
can't be black characters (and there are people from other areas in Middle Earth who are dark-skinned), but even if a black elf can't be explained, so what? It doesn't have to be another front in the culture war. Obviously the people making the show can still be criticised for doing things that don't work or fit the spirit of LOTR, but a lot of this is quite hysterical and goes way beyond that, into some kind of moral crusade, they're trying to brainwash us with woke propaganda, it's a betrayal of Tolkien, they're trying to deliberately insult the fans, etc etc.
And I only said
some of it is veering into white supremacist ideas. But when you see white bearded American dudes talking about 'European culture' and 'Western Christian values' being destroyed, and sinister forces who control the media, it's pretty obvious what they're getting at.