I also ought to point out that Star's non-Marco love interest option who HAS been (mostly) treated with respect is Tom, and he DID seem to make Marco jealous in some way when he first showed up. But Star is flat-out over him, and he seems to have stronger ties to Marco these days anyway, so he's not a direct analogue to Jackie, either.>There hasn't been any indication that they should break up.
This will feel a bit like reading tea leaves to me, and there's a great big catch that I'll get to later, but I've noticed that in both of the Season 2B episodes where Jackie and Marco interacted, there was some point being made about how they didn't quite "click." There's Star and Marco's "Our second-favorite song!"/Jackie's "They're playing my favorite song!" in Just Friends, and there's the failed mind-reading (maybe to be contrasted with Star and Marco on the balcony in Blood Moon Ball?) and "Jackie is no criminal" scenes in Starcrushed. Little but frequent things that show that Marco's not in sync with Jackie as much as he is with Star.
But here's that catch: little misalignments like that are not good reasons to break up (unless you're a Seinfeld character)! They're trivialities that can and should be dealt with by communication and compromise. So far, there IS nothing about Jackie that would give Marco reason to break up with her.
Of course, "the heart has its reasons which reason knows nothing of," so it probably will come down to how Marco feels. Could he, like Star, realize that his feelings for her go way deeper than best-friendship now that he's forced by losing her to sort those feelings out, (just like she did about him?) It's a possibility, but there's no hard proof for things going that way. Yet.
(And, of course, there's the possibility that Jackie might dump Marco for being more devoted to Star than he is to her - there COULD be cause for that option, but I don't really want to think about that too much because it's a really ugly possibility.)