5) Whitebeard takes in Ace believing, that being Roger’s son, he was destined to be the man Roger and Joy Boy are waiting for;
Whitebeard found out the hard way Ace wasn’t the one, but knew someone will certainly arrive as per Joy Boy’s prophecy. He bowed out stating as much, ending the old era and setting the stage for Joy Boy’s inheritor.
Whitebeard wanted to save Ace, but as jarring as it is to realize after 400 chapters, it wasn’t just because he was family. He believed Ace to be the one to open Wano as Joy Boy. Certainly, he wanted to avenge Oden, but knew it was pointless. Only Joy Boy could. Unlike hot headed Ace, he had a bird’s eye view of the next 20 years, Will of D. and One Piece.
As the leader of the era, he made the unemotional politically and historically sound decision. Losing crewmates in a Yonko war that’ll probably drag on for days wouldn’t bring Oden back. And as per Oden and the prophecy, only Joy Boy (or Ace in Whitebeard’s belief) could achieve Oden’s dream 20 years after the time. Ace wasn’t ready.
Yamato read Oden’s journal. She obtained the same knowledge Whitebeard had. She knew Joy Boy would appear on this day on Wano. She’s been waiting for Luffy. The man started a revolution while shackled in an underground prison, of course it’s his fate. The D. storm is coming.
*Theory by Akai2
