Outgrown her? Star is pretty much his other half at this point. I mean seriously, look how he reacted to her leaving. He's just being dumb and oblivious. The whole thing is being set up to show how completely stupid he is when it comes to articulating his feelings. This is the same guy who spent 10 years pining over a girl (ironically, the same girl he's with now). He already acknowledged that feelings change and that he may have placed Jackie on some pedestal.
Its why this show hurts so many autist feelings. Its a more realistic take on how guys like Marco operate. Spending years pining from afar about a dream girl, while being completely ignorant of any girls that are pining for him.
But... it looks like he's going to choose his "dream girl" at the end of the day anyway. And that's gonna possibly play into a heel turn from either of them, delaying Starco even further.
But maybe not. Maybe all the signs are there and we're massively overthinking it. But the whole "Star's heart breaking" imagery in the trailer was a little too much of an indicator of where Starco is about to go.