I have a lot of thoughts on this series. I'll share one:
Satou's death is probably the worst thing that can happen to Shio, but not for the reason you might think.
Lemma. Satou and Shio's relationship was doomed before Satou's death.
Evidence. We've seen that Satou takes extensive measures to keep Shio to herself, because she believes that Shio doesn't need anyone else. She refuses to give Shio the support of the government or her blood-related family. It's for the same reason that Satou would ultimately be unable to give her away to her true owner—herself. Recall what Shio tells Asahi in the apartment complex: “I’ve decided to live for myself.” A Shio that lives for herself is not one that can accept Satou’s vision of love.
Ironically, Satou's martyrdom is the only way for their relationship to continue—for Shio to remain deluded by an idealized ghost. And this is the greatest tragedy, because it means that Shio will never break free of the cage that is Satou's love.