They never dated, lol. As far as I remember, Banri got hit before he could even confess. Plus even if they dated, the fact that he forgot his love for Linda quickly just because of the amnesia must mean his love for her was not that wrong. Whereas Banri still remembers his love for Koko even after forgetting her (when he got his memories back). So yeah, the ending just proves who he really likes better. And in real life, you don't always end up with you childhood love. There are many circumstances that would play around and it's not always subjected to childhood lovers or who liked A first. That's life and I think that's the realism Golden Time wanted the readers to know.
I hate the ending of this so much. Should've ended up with the ex