The idea of them breaking up down the line, while sad, does make a bit of sense.
I can totally see them getting into a catastrophic misunderstanding close to the end of their uni days, and call it quits. They might try dating others, but they'll eventually meet up at a mutual's party a few years later, maybe Junseo's wedding, where they'll reconnect. The two will probably have had spent a few years apart where they'd have matured some. Learn from their mistakes, or at least try to.
They might brave the idea and start out as "just friends" again, but they'll end up hooking up a month or so later. It'll be on-again, off-again for a year or two because old habits die a tad bit too hard, but I can totally see them just saying fuck it and having a shotgun wedding somewhere after they realize that despite it all, they really do love eachother.
And thus concludes my mini-sequel-sequel fanfic lol