While I totally agree that believability is very very important, I think that even more than that narrative coherence must be enforced.

Since in this whole series the main characters never die, well, they never die. You can't expect them to die just in the last book, it would be stupid. Either they meet their fate constantly throughout the series, or they don't at all.

This is a mistake Araki made in the third series of Jojo: all the characters survive through dozens of fights, and then a lot of them die just in the final chapters. It doesn't make any more sense than them not dying at all, and it is not narratively coherent.