Most fantasy characters can be played by anyone, their skin color doesn't matter. Most fantasy property was written in an age where black people were second class citizens in many countries, including the US. Obviously we should move on from those times and give leading roles for black or Asian actors that they would have never got decades ago.
Black Panther is not comparable because he was written very specifically to be black, their place in comic book history is defined by their skin color, therefore the role can't be played by a white or Asian actor. The little mermaid is perceived to be white not because it's relevant to the plot, it's because the character had to be white in 1989 due to white being the default skin color for any leading role in Hollywood at the time. Now the tides have changed.
And Little Mermaid, the Disney musical, has almost nothing to do with its European/Danish origin. The most well-known song of the musical, which won the Oscar in 1989, is a Caribbean song, performed by a black actor, who played Sebastian. Disney already made their own thing and Americanized the tale in 1989, all their are doing now is that they are updating it to what the public desires in the 21st century in the US.
I mean, this sure as hell ain't "European" or "Danish" in any sort of way.
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That being said: the Disney live action remakes are useless and shouldn't have been made. The animated features are superior, but not because they have white actors/characters. So all of this debate is pointless, the skin color is NEVER the issue. The issue is with how unnecessary it is to remake great animated features into mediocre live action movies. The Jungle Book was the only remake which was an actual improvement, every other remake was completely pointless.