The Lucifer and Biscuit Hammer/Headscratchers

Why didn't the author just finish the series with the last page of chapter 63? It would have ended on a high note and kept the exact right amount of open-endedness.
Because chapter 65 ended on a higher note and provided both open-endedness and closure. Also, "growing up" both physically and emotionally is a ridiculously important theme to the series, so leaving the cast hanging where they were would have actually worked against the series. As Anima said, this was just an episode in their lives, albeit an important one, but they have their whole lives to get on with now. Go read Psycho Staff and you see the same themes cropping up, so they're obviously important to the mangaka: being a hero isn't something you can do forever, eventually you have to grow up and be an adult too. And it turns out you can be happy as an adult, happier even than you were as a hero.
 * What can be higher note than a duel till KO between the two love rivals of some 50 odd chapters? Pairing all the characters together and having a beach ep?
 * They weren't love rivals. They were just rivals.