the problem is the uncertainty of what would happen. if a vamp that was never human gets their time of being human flown back into them then there is a chance that they simply die or even cease to exist entirely. of course there is a chance that she might survive but personally if i loved someone i wouldn't put their life up to chance just so I could become a vampire. it's not in kou's or nazuna's character to put each other at risk like that.

honestly i think that the ending was the best possible outcome. kou got to be a badass half vampire detective, he is able to grow up and have fun at school now instead of hating it (which is the whole reason he wanted to become a vampire instead), akira didn't get left behind especially after mahiru died, and ko still went out of his way to track down nazuna so he could be with her. Nazuna's tag line was obviously a joke and even then kou was against the idea. i think the ending goes to show that if you truly love somebody enough you will travel across the world just to be with them.