First reason is because of Big Mom’s death. I know it’s stupid, but suppose if Big Mom dies for some unknown reason, Katakuri will lead the Big Mom Pirates just as Marco became the captain of the Whitebeard Pirates after Whitebeard’s death. Katakuri will be in the same spot as Marco and will lead his Pirate Crew.
This made me think that after the death of Big Mom, the power of the Big Mom Pirates will decrease drastically and most likely other Yonkō will have their eyes on them. It will be a similar situation like when Whitebeard Pirates lost their captain, other Yonkō tried to take out his territories.
And they will probably want to take over Big Mom’s territories because of the Road Poneglyphs. Moreover, the all the homies, chess soldiers will die with Big Mom, and the allies who joined the Big Mom because of fear will leave their crew.
Then Big Mom Pirates will be an easy target for any other Yonkō, so what I think is that Katakuri will tell Luffy to help him or Luffy will help his friend in his greatest need. This could be a great build up for Katakuri joining the Strawhat Grand Fleet.
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