I believe there are only two realistic ways for a Tomstar breakup in the future: Tom gets sloppy and slowly reverts to his older self, or Star and Tom having very different views about the Monster-equality thing.
I can also imagine other scenarios that would be less bad for Tom’s character. Like, could Star and or Tom (better if both of them) realize that they were in love with the idea of a relationship, more than with the other person? Tom put a lot of his self worth in being able to get back with Star, it was what first pushed him to change, and something he could never get over; Star was hurt by all the events with Marco and wanted to change her life as soon as possible, and might have seen more appeal in the idea of a relationship with Tom, something that would “occupy” her feelings, rather than in Tom himself?
Obviously I’m speculating hard here, the show is nowhere near to implying things like this yet, but Monster Bash did show that they’re not exactly on the same page, relationship wise: Tom is very focused on the physical aspect of dating, Star seems to just consider Tom as her boyfriend because that’s who he is. “Sorry feelings, I can’t think about Marco right now, I’m with Tom!”.
I don’t want to offend any Tomstar shipper with this, just saying what I think the show MIGHT be building up to.