and still things fall apart, because in that case the story should never have introduced the fact that she could just be resurrected. To bring her back is missing the point? Then Fuyumura should have had more to say than "I'll think about whether it's right to resurrect her" moments before she ceased to be a character in Sanda and became a plot device with barely any screentime until the end of the manga.
There's also this little known narrative trick called Checkov's gun, don't introduce something into your story if you are not going to make use of it. The reindeer santa special power could have just been a vehicle creation ability without the resurrection and that would've been that. But it wasn't.
The moment the story introduced a way to actually bring Ono back to life is the moment the themes needed to incorporate reasons for not doing so that are better than simply hoping we'd forget about it. Which is what happened.
You really hang up on a character that been dead since the story started and at that point had ceased to had any relevance.
The part you seem to decide to ignore is Fuyumura gone over Ono's death in that arc and come to understand loss, even if the analogy was more "social death" as Ono simply grown as Fuyumura remained a child and so they ceased to have that much in common (like two children that are 2-3 years apart and eventually cease to interact as one grows into a teenager and moves into another social circle) and that was the whole point, even her disease was entirely "she grow up too fast" ... its not that the analogies are even subtle, they kinda really obvious and on the nose.
Also you are ignoring the Reindeer Blood on its own only heals wounds, the possibility of resurrection is explored but also its
full consequences since there are moral consequences, lets not forget this
WAS NOT forgotten, go check Chapter 74 as you see her in front of Ono's "grave" with Santa thinking about it, I believe the anime goes into that more.
It seems you just wanted to do it as the whole purpose of the manga was bringing Ono back that Santa cant do at first, there is a history of Santa keep failing his promises to Fuyumura but that is also thematic, children make promises they cannot keep, its a story of growing up and Sanda and Fuyumura simply accept Ono's death as part of life.
Its not really a flaw of the story, it would been maybe if it was completly ignore but it was not ... Fuyumura praying at Ono's "grave" is symbolic of acceptace, its not she forgot but that she knows that one should be act like God and bring back the dead.
