Let’s talk about black Ariel. First, my credentials:
Disney’s 1989 The Little Mermaid was the first legitimate VHS tape I ever owned. It was gifted to me as a Christmas present because I had been so abjectly in love with the film when my grandparents took me to the theater, and I proceeded to watch it at least once a day, every day, for nigh unto all time. The tape developed a noticeable squeak by the end of year one. My parents probably still twitch if any portion of the sound track comes on within ear shot. I knew that film forwards and backwards, and to this day can still quote the whole thing at you, as I heard it and learned it when I was five. (That is to say, there are places where the script turns into nonsense syllables.) I did not have all the Little Mermaid merchandise, but everything I owned was Little Mermaid. It was my monochromatic color choice before I found black. My entire social life centered around the fact that I was the chief authority on the film and its source texts. That made me god-king of the playground, and I was a fierce and benevolent leader.
My first aesthetic, philosophical crisis as a child was when, at the age of eight, I realized that Beauty and the Beast was an objectively better film, and had to decide whether that made me a traitor. I concluded that I could acknowledge the ways Beauty and the Beast outstripped its predecessor while still faithfully loving the other one, and thereby achieved, at the age of eight, levels of maturity the internet would still lack a quarter of a century later. Good job, humans.

And, frankly, humans behaving like a stupid sack of salted rocks is why you’re getting this rant. Because, excuse me, Ariel can absolutely be played by a black woman. White skin is not an intrinsic element of the character. Red hair, and the color of her tail (invented specifically for her by the Disney animators, a fact I learned while despairing of ever finding its match in a crayon box while coloring in pictures of her) are the only pigments that are in any fashion intrinsic to Ariel. She’s just white because Hans Christian Anderson was and Disney, in 1989, didn’t have the guts to handle fragile white-folk pearl clutching in 1989. (To be fair, it’s not that they have those guts now. They’ve just figured out a a different marketing model from the one they were operating under then. Like you do, over the course of thirty years.) In fact, the creators were pushing boundaries by making her a red head instead of a blonde. Everybody at the time thought mermaids had to be blond, and the creators rebelled. Ariel’s been a pigment-boundary-pusher from the start. (My source: The DVD extras on the special anniversary DVD edition my sister gave me for my birthday to replace the poor, squeaky tape. I promptly watched everything.)
None of what I have already said, or am about to say, actually needs to be said, because the people having a panic attack over this, while entitled to whatever emotional hyperbole they care to indulge in, ought to be keeping it to themselves and sparing the rest of us their idiotic tantrum. But you don’t spend the formative years of your childhood obsessively trying to figure out the land version of a sea-witch who can get you access to another world where you fit in, then let this nonsense go unchallenged. So, let’s instead flay the stupid, point by point.

Ariel is white in the source material.
Did you really just make an argument that Disney should be faithful to the source material? Are you out of your head? Have you missed the last century of film production by that company? Do you know the ending in the source material? (It’s better, right up until it’s worse.) Take that stellar rhetorical point scoring of yours and eat matches.
It doesn’t make sense for there to be black mermaids in Denmark at that time.
First of all, stop learning your history from faux-medieval fantasy novels, and your idea of population migratory patterns from antebellum anti-abolitionist hacks. Second of all, what actually doesn’t make sense is a white mermaid in the first place.
Let’s set aside the whole truckload of problems with the notion that you can handle the existence of mer-civilization and talking fish, but not black ones, and look at this from a reasoned, biological point of view: It makes absolutely no sense for Ariel to be white. Assuming her pigmentation on her human parts are driven by human evolutionary processes, she’s black, and I don’t mean “Acceptable for Hollywood light-skinned black.” Mutations that have created the variety of pigmentation present in human phenotypes are driven by a variety of environmental factors, most notably, interaction with the sun. This is not a pressure present in people who live at the bottom of the ocean. I don’t know how mer-folk are getting their vitamin D, but it’s not through direct manufacture via sunlight. Maybe, maybe some random mutation got preserved for another reason and Halle Bailey can swan in as a perfect pigmentation match. There’s no chance you’re winding up at a “Plausible to hysterical white people for the north sea” complexion.

And if their pigmentation doesn’t follow human evolutionary patterns, but aquatic ones? You’re stretching to either the beluga whale, or the great white shark, as your paths to a white mermaid. Whales actually do routinely interact with the surface, so you have some legs on a north sea-based argument there, though siren stories tend to trace mermaids south, not north. As for great whites? They’re mostly kind of a dull gray, with white bellies. That sure is one attractive mer-fish you’re designing there.
This is all just a campaign to make liberals happy.
And, so what if it is? That’s not an argument so much as a pile of scrap tin with one leg. Liberals have money. They buy things, and boycott things and like to see the world as they understand it reflected to them. You see, they’re human. Unlike mermaids. It is totally within the rights of an old, global megacorporation to decide that seeking out the dollars of “liberals” is worthwhile, even if if means making you, tiny numskull cretin that you are, uncomfortable. Go back to sucking those rocks from earlier for comfort if you need it.
This is more proof that political correctness has gone too far and people are being forced to comply with new groupthink standards and pass virtue tests.
Yeah, Disney looking at the massive success of Moana and deciding to make sure they don’t shut themselves out of that global audience when they once again prostitute a beloved element of my childhood for untold lucre is totally a return of Hollywood Blacklists and McCarythism. Because we’re having congressional hearings where we publicly lambaste people just for being friends with white supremacists and then rendering them unemployable for the rest of their lives, right? No? What are you whining about?

The idea that you can defy the supreme legal authority of your realm, treacherously make a pact with a known national enemy, then seek to form a binding alliance with a foreign, hostile power, and all your dreams come true as a result, is an intrinsically white narrative and should be portrayed as such.
Nobody made that point. But you could, and I’d still respect you after. My response to it is, simply: Sometimes art has to be the vanguard and drag reluctant reality behind it. Let’s open the doors of privilege and consequenceless-recklessness aristocrats of all races.