Numinous

I think Sabo is dead due to his character type. Like Bell-mère or Olvia, he was meant to die to shed light on another's characters personality and actions. And, like them, he has no major purpose in the plot anymore. His introducing character is already done and what would make him stick around? Ace is dead and Luffy won't go back to Dawn Island for a long while, so there's no longer that feeling around the character. Furthermore, Sabo being alive would cheapen both his and Ace's death scenes, making Luffy no longer the only one of the three rascals who survived.

Also, I think that, if Oda wanted Sabo to be alive, he'd play one of his tricks (like Railegh being introduced way before Sabaody), but, as far as I saw from Dragon's scenes, I haven't spotted Sabo anywhere.

One last thing, just because Bon Kurei is alive doesn't mean Sabo can be. If you notice, both instances where Bon Kurei was thought to be dead it was played as scenes of sacrifice, not of morbid death like Sabo.

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