And yet, we do have the concept of character copyrights, which says that you cannot use someone else's copyright characters even for your own entirely original work.
So my point is that the very existence of character copyrights means that that reasoning cannot be entirely valid.
In particular, I would dispute your reasoning in this step: "How much of the original work does it copy? In this case, very little. Just the appearance of the characters. All the footage is original."
But the copyright that we're talking about is not a copyright on the original work, it's a copyright on the characters. And then the question becomes "How much of the original character did you use?" and the answer, is, essentially, 100% -- because a character either makes an appearance in your story, or they don't. (Especially in this case where the whole short film is about these characters.)