Well yeah, but a huge chunk of the audience loves romance drama, twists and plots and episodes about that. Romance genre stories sell a lot and thinking about relationships and love is a big part of people's lives.
There's kinda two big plots intertwining; one is Star's journey to become fit to be Queen, which includes all this political intrigue and fighting the forces of evil and trying to grow as a person and learning to be less reckless and listen to advice and duty and responsibility and so on.
The second is Star's relationship to Marco, and them progressing from annoyance to friendship to crushing to awkwardness to figuring out how to deal with relationships and love and balance that with friendship etc. How to be a good partner and not selfish, how to be honest to yourself and others about your feelings, all that.
Maybe there's something about Marco and other side characters growing, too.
So, even if Starco would become canon, they wouldn't suddenly drop the romantic relationships aspect of the show and only focus on kickass fighting, but instead there'd be some problems in their relationship, maybe they'd consider breaking up, and so on.