I think MorningMark had the right take on the difference between Jackie and Star. If Marco stayed with Jackie on Earth, his future was already guaranteed. He has money coming in from Princess Marco merchandise and his parents love him to death and will do anything for him and make sure his needs are met. He has excellent grades in school, and he's a strong student, so he can get into any university to get a degree in whatever he wants. Thanks to finally coming out of his shell, he has the perfect girlfriend who loves him for who he is in spite of his flaws, and his circle of friends has grown from people laughing at him wearing a helmet in the locker room showers, to his house being full of fellow students when he hosts an end-of-the-year party.
With Star, he has no future. Not in the sense of "he's going to die" but in the sense of there's absolutely no certainty to what's going to happen next. Star's future is uncharted territory, even beyond becoming the next queen, because she has a spontaneity about her actions.
What the show failed to do, I believe, was express not only how Marco's nightmare back in S2 was affecting his judgements, but also failed to ask the question if there's a difference between "contentment" (Jackie) and "happiness" (Star), the risks involved in "blind faith" (Star) and "security" (Jackie). Most importantly, I think it would have behooved them to have Jackie state something along the lines of, "I love you for who you are, just as you love me for who I am," and Star state, "I love you now and whatever you might grow up to be, just like how you feel about me even if I have to become a boring queen."