>>116632301Weird as it might sound, the fact that Jackie was actually meant to be royalty and from Kingdom right next to Butterfly's, and that Star's comments about the mermaid people were prejudice, means that Jackie was probably going to join Marco's journey through Mewni.
Adding another fact, that during their date, Jackie mentions she loves readying about history, would mean she was surely going to know a lot more about Mewni's history, probably better than Star by a mile and therefore humilliate the main character in being a Princess.
These theories would be a good reason why the authors (Daron and Bisignano) and/or writers were convinced, or decided, or had a change of heart, to remove her from third season.
