I interpreted the story differently. Makoto's attraction to Kotonoha was always mainly physical: when you're staring at someone across the train, you're not thinking about romance, you're thinking about her big tits and how much you'd like to fuck her. But once it becomes apparent that Kotonoha isn't ready for sexy stuff, and he realizes there's another girl who's actually spending time with him and interested in the stuff he wants to do... then it becomes clear that Sekai is the winner. And then Kotonoha tries to step up her sexiness to win him back, but it's too late for her. You snooze, you lose.
Besides, the only reason Kotonoha doesn't backstab her friend is because she doesn't have any friends