It's not matter of falling out of public eyes, you think Alfonso Ribeiro is an "has been", that is irrelevant. A company is selling something that someone else created, the someone one can be a nobody, the point is, "you" can't sell something that another person owns without his consent or giving said person his cut. That is why he is suing.
M.J may not have created the moon walk ( in fact he did not ) but I bet that if a company was selling the move without the Jackson's consent, they would be suing too. Anyways, I probably used a bad example.
As another poster said, choreography can be copyrighted end of story. It's not hard to understand, it's like any other work of art. A better example maybe : Fortnite is selling a skin, blue and red, with an insect design on the chest and big white eyes, it's just colors and an insect...right ? Do you think Marvel will get it slide ?