I honestly think that Eclipsa really is not Mewni's legitimate queen.



It was proven that Eclipsa left the throne to live happily with her family and thought that without a queen the war that her mother left would be over until the high magical commission adopted the daughter of a peasant, Festiia, the true ancestor of Star, to take the place of Eclipsa as next queen, starting a new legacy of the Butterfly family.



Therefore, Star's family did not take Eclipsa's throne away from her family, so things should not have changed if it was her decision to stop being queen, so River and Moon would be the legitimate kings of Mewni, Star would be the future queen and she should not have given the royal wand to Eclipsa if she left it as a gift to her ancestor Festivia for a better life.