Yes the exact point is that Andersen isn't the original story. Creators change things when adapting stories.
Trying to delineate which changes are justified and which changes are off limits is ultimately all arbitrary. The only guiding principle is that when you are working with a previously established story, you usually try to stay "faithful" to the "core" elements of the story such that it is still recognizably the "same" story.
The Little Mermaid is a tragedy which ends with her turning to sea foam. Andersen changed the ending, which is arguably a more core element of the original mythos of this fable than any description of skin color.
Why is it that just because this movie is coming from Disney that they are bound to their previous portrayal of Ariel's physical characteristics whereas Disney itself is not bound to retelling the story exactly as it was in Andersen's book or in the original fable (where she turns to sea foam at the end)?
What is the justification for the extremely specific set of creative limitations you want Disney to adhere to here that did not exist on Disney in 1989?
Look I'm not saying creators get to do whatever they want. But I'm saying that when we have objections to things that are changed, those objections need to be grounded on a principle. It's one thing to say you just don't like her as Ariel (and most people will interpret that as you don't want a black girl portraying a character that you grew up thinking was white) but iyou're saying there is a specific reason why this change is wrong. And those principles need to be consistent.
I am saying that in the end it seems we all agree with the idea that something should be "faithful." What we arguing about is which details are integral enough to be core such that violation means an adaptation is no longer "faithful." And then we end up here in a situation where we are debating the relevance of SKIN COLOR of HUMANS and whether it applies to FICTIONAL MERMAIDS.
People here have been giving various extended defenses of why her skin color is not an important core feature of her character such that changing it makes the adaptation of the film "unfaithful" to the original (whether you consider that original to be the 1989 cartoon, the Andersen story, or the original fairy tale).
In other situations (making Black Panther white), changing skin color would be a change that makes adaptation unfaithful to the original. And, despite claims of "reverse racism" it's not about the direction of the change, it's about the story. (SEPARATE from all of that is the related but closely tied issue of minority representation in media, which generally holds that changes one way but not the other are legitimate.)