Shibuki seems like a alien element in the whole saint-demon lord cycle.
If it was "proper" cycle then Safi would have taken the holy sword and killed demon king. Then something would happen to her since the church picks up another saint.
But in this cycle Shibuki took the sword.
But maybe that's the "proper" cycle?
God allows external influence so the story unfolds as:
1) Guy takes a sword and protects the saint
2) they defeat the demon lord but the party dies in the process
3) Demon Lord curses the guy so he becomes the next demon lord
4) God takes the saint who is now wholly devoted to him since he's the only one left after her party dies
5) God/Church choses another cutie to become a saint a the cycle restarts