Let's go back to the conversation between Star and Glossaryck in the Grandma Room.
S: So... no magic means... no Marco?
G: Actually, no magic means no... Glossaryck. Marco will be fine. The magic will put him back where he belongs.
S: On earth. But that's not fair! Plus, that doesn't even make sense. The magic put Marco in the lint catcher at the Mewni castle...
S: ... and my mom on Pie Island! ...
S: ...My boyfriend Marco, he is missing in this picture. Can you please just add him in there...
S: ...He belongs with me!
G: I think the tapestry is done.
Well, there are... several possibilities here. First: everything plays out like it was implied in this episode. When Star destroys magic, it puts Marco back on earth, and the two are forever separated.
Second possibility: Star was right in saying "He belongs with me." The magic puts him "back where he belongs," which is with Star on Mewni.
Third possibility: the magic puts both Star and Marco back on earth. The Mewmans and the monsters are now forced to look after themselves. They eventually settle on a new system, most likely a form of democracy. Meanwhile, Star and Marco live out the rest of their days on earth together.
Fourth possibility: the tapestry was wrong. Those tapestries are not entirely accurate, as we saw in Moon the Undaunted. (Moon did ride Lil' Chauncey out to meet Toffee, but she wasn't standing on him when she cast the spell... and Toffee didn't look quite like he does in the tapestry.)
Also, here's some more commentary on that scene:
Why did Moon end up on Pie Island? Well, what sets the Pie Folk apart from everyone else? They are huge scammers. They can scam anyone out of anything. They can even scam each other out of 45,710,412,242 pennies (or $457,104,122.42). So if Moon is Pie Folk too, it only makes sense that she can run a scam, even against her own family.
Just because Marco is out of the picture completely doesn't mean he's out of the picture completely. While some queens' tapestries basically depict their whole lives (such as Celena), others' tapestries simply depict a defining moment in their lives (such as Moon). It looks like Star's tapestry is the latter, and her defining moment is destroying magic. That means the rest of her life is not shown here and could very well end up being with Marco. Nothing about Moon's tapestry screams "River," yet she ends up marrying him. Could be the same deal with Star and Marco.
There's also the fact that a major theme of this show is "free will vs destiny" and how Star continues to buck the trends. It's not out of the realm of possibility that she finds out some new information, which leads her to outright reject her tapestry. But it also seems like Star's destiny IS to buck the trend, and what bigger way is there to do that than to destroy magic?