Shinako and Rou are frustrating characters, but that's the point. If you're frustrated by them, the mangaka accomplished their task. Every character is tied to the past in some way, to past feelings. Some overcome them, some don't. Those that do, find happiness, those that don't, are doomed to linger in purgatory and suffer.
While Rikuo and Haru will spend a happy lifetime together, Shinako and Rou will regret not having moved on. Especially Rou. He sacrificed everything for the sake of his childish obsession. The manga itself pointed it out nicely.
All the other characters got satisfying endings, as well. Even if it was a bit too saccharine, I'm glad no one was screwed over. A manga shouldn't be too realistic sometimes.