I thought this one was a bit depressing.
I mean, how often do all of you guys think about the future? We'll all die someday, and the Earth will eventually be destroyed by some cosmic force like the sun expanding or a meteor.
But Mokou and Kaguya? They'll keep on living. Even after the end of everything, after all of the stars have burnt out, and after the heat death of the universe they will live because of that curse disguised as a potion. Mokou is already over a millennia old, Kaguya is who knows how much longer than that.
I'm sure they'll each have a few centuries of nothing but attempts at suicide. There will come a time when they think that they've lived long enough, too long even, and just give up. Death of the body, the elixir may prevent, but death of the mind and soul cannot be stopped.