Despite making a whole heap of good points, I think you're fundamentally not engaging with the two most important aspects of the issue: acquaintance and world-building.

First, world-building.

Like many people, without quite saying so, you're presenting an argument that seems to be assuming that because fantasy is made-up, that anything goes. But that's completely wrong. The settings need to be relatable and predictable. They very much are like something: it's constrained imagination, imagination of what a possible world could be like.

If you're going to build a world based on the history of northern Europe, you can't just randomly, say, make the King black with no explanation any more than you can randomly stick a laser rifle in with no explanation. Now, if you say: I'm bored of worlds like that--sure! Fine! I'm kinda bored of them too. But if you give the King a Tesla when everyone else is on foot or horseback, you'd better have a really good explanation or you're not writing high fantasy: you're writing a spoof.

So if you take a carefully crafted setting that is very much like its own thing and you randomly race-swap a few characters without fixing up the setting so it makes sense, there's a problem. You broke the rules of the fictional world. There's an easy way to compensate: fix up the setting. Done! No problem.

But it's certainly not the case that "While casting a black person as George Washington may break the viewers’ suspension of belief, this logic works perfectly in the fantasy realm." If everything about the setting is screaming at you that the George Washington character should be Asian (e.g. all other people and cultural characteristics are reminiscent of east Asian cultures in the real world), and the character is white, or black, or Arabic, there's just as big of a problem. Fantasy worlds have their own rules.

Now, the rule could be that everywhere is racially diverse, and nobody notices. As long as you don't think about this too carefully ("How long has this been the case? How are the different appearances maintained? Why doesn't everyone look multi-racial? Is there rampant anti-miscegeny? Or does heritability of physical traits work totally differently here?"), it's awesome. In movie settings, you generally don't think about things too carefully, so you're good. If you're writing a book, you probably want to have an idea in mind. But, anyway, you can totally do multiculturalism in fantasy, or you can do a different race and culture than whatever the typical is (fantasy Maori samurai--why not?), but you can't make it not make sense.

Second, acquaintance.

We don't live in a color-blind society. This means that when we get to know someone, we get to know them including their immutable characteristics like skin color and sex.

People get to know fictional characters, too. You can make up whomever you want, but if you say you're talking about someone they actually know, even if the person they know is imaginary, you can't really change very much without being really really explicit that you're changing things. Even then, people might not forgive you. If someone says, "Hey, come over and have lunch with us! Come quick! Your mom's here!" and you go over and find that some other woman is there, you're probably not going to like it, even if that other woman is nice, even if she has an interesting story herself, even if she's kinda the same age as your mom. It's not your mom, and that matters.

So if you tell everyone you're going to make a new movie about Elsa, except "Elsa" is "El Sando", the nickname for a short Filipino boy who wears a characteristic undershirt and likes to eat ice cubes, it's entirely unreasonable for the audience to be really upset. Not because you can't tell an awesome story about a short Filipino boy who likes to eat ice cubes. It might be a fantastic story! No, it's because you lied to the audience: you exploited their affinity for someone they knew in order to get attention, and then switched out who they knew for someone else.

For instance, take "Galadriel" in Amazon's Rings of Power. She's white. Galadriel is supposed to be white. She's blonde. Galadriel's supposed to be blonde. Anyone who really likes Galadriel, however, is absolutely adamant that there's no *#@%&ing way that that's Galadriel. She's short, young, impetuous, fights with swords using gear with the symbol of Feanor's house...it's NOT MY MOM. A lot of fans feel betrayed because Amazon promised Galadriel and basically lied: they reused the name, and a few concepts, but she's not the Galadriel we knew.

Until we live in a race-blind society, same deal with race-switching. It's not one of the very most important features of a hero, but it doesn't feel right if The Rock plays Merlin, because even with all the different interpretations of Merlin, The Rock's general appearance falls well outside that.

Now, cosplay is different. If you're going for homage, you can be an awesome cosplayer without a perfect skin color match. If you're going for accuracy, wellllll, then it's a problem. If you're going to cosplay as J'Onn J'Onzz and your skin isn't green, it's a bit jarring. Most cosplayers do the green skin thing. As a cosplayer, you can't redo the setting, so you have to rely on copying as many of the distinguishing characteristics of the character as you can. Linda Le is a fantastic cosplayer, but she's not above using makeup to change her skin tone when it helps to get the look right. (But she's also awesome at not changing her skin tone and still nailing a lot of characters.)

So, anyway, (1) consistency of setting matters, and (2) in a non-race-blind society where "looks like me" is thought to matter, you can't race-switch someone you've gotten to know and have nobody blink because race is (thought to be) central to identity.

That said, I think the dearth of non-European-setting stories in popular culture is tragic. People all over the world have fantastically rich traditions with deeply engaging stories, and we are all dreadfully impoverished by having White People Use Swords And Spells, Episode LXVIII or Mostly White People Comic Book Adaptation #152. But you know what? If you switch out "White People" for other races we're still almost as impoverished. I mean, the Lion King had a frickin LION as the main character...and the bad guy!, but it wasn't a particularly different story from Aladdin or Beauty and the Beast. If it's the same story, you can switch the race or the species and...well...I'm sure it was great for all the lions of the world to see main characters who look like them, but honestly, it didn't do much for story diversity.

For instance, I always loved the Anansi stories I read as a child, even though they were not entirely authentic renderings of the Ghanan originals. Why don't we use that as a starting point for fantasy stories in the United States? Or use the Romance of the Three Kingdoms? There are a myriad of choices. Our species has invested so much creativity into our stories and our fantasies and we're barely scratching the surface in popular culture in the United States.

(Note: non-Western stories have plenty of stereotyping of fantastical races/species. Also, having a species-level threat (e.g. orcs vs humans) is a very effective way to overcome any racial animosity in people. See Star Trek for perhaps the clearest example. So I totally don't buy your comments there. It could be used to stoke racial animosity and stereotyping, but its even easier to use it to do the opposite.)

Anyway, I don't think high fantasy needs to be white-centric (just in an appropriately-built world), but it can't help being Eurocentric because that's where all the cultural details come from. And while there are still plenty of good stories to tell from that perspective (how could there be otherwise?--the space of all possible stories is limitless!), I don't think the low-hanging fruit are there.