I think Yuri and Jaerim can last forever. At first they were unstable but that was because Jaerim was being too much of an idiot and then it was because Yuri's personal problems and trauma, but Jaerim finally admitted to his feelings and became honest and after they addressed Yuri's trauma and solved it, I think they became a very strong couple than can get through everything and anything. Now, Aerak and Heeno, I don't think they are a good couple because Aerak refuses to tell Heeno about their past and be truthful about himself. He's always putting up a front and is never honest nor does he show his true self, so that thing about them growing old together isn't believable at all
I really do believe their relationship won't last long, nor jaerim and yuri, nor aerak and the dickhead guy (sorry) for me, both the couples seem really too much instable, you know it feels like if there's a little problem showing off, one if not both of them gets overly deressed