Oda has foreshadowed for years the one who will deliver the final blow to Kaido and it’s Roronoa Zoro.
This is why he will finish off Kaido:
-Let’s start by saying that no one is claiming Zoro will 1v1 Kaido. It is an all around effort to kill Kaido.
-Zoro and Kaido represents the Tiger vs Dragon, respectively in Chinese Mythology in which the Dragon holds power for a long period of time in a land in which the tiger eventually challenges him.
-Zoro = Ryuma; Kaido = Flying Dragon. The legend of Ryuma slaying a flying dragon is constantly stated with Zoro around.
-Orochi is NOT a flying dragon let alone a dragon. He is an 8-headed snake that doesn’t fit the Ryuma narrative of slaying a flying dragon stated back in Thriller Bark. Furthermore Orochi is too weak to be Zoro’s final enemy, as he was about to get 1 SHOTTED by a nerfed 2 sword Zoro’s 720 phoenix.
-There are 2 Yonkos in Wano. Big Mom started her rivalry with Luffy since Fishman Island when the Straw Hats freed them from her control. On top of that Luffy defeated her strongest child. Furthermore we still don’t know what happened to Jinbe. He could’ve lost a limb or be dead for all we know. That gives Luffy his reason to target Big Mom specifically. So Luffy will get his shining finishing moment against Big Mom.
-Oda would not make Hiyori offer Zoro the only blade that ever scarred Kaido if it meant his final legacy defining moment will NOT be slaying Kaido’s impenetrable body(with help of course).
-Zoro NEEDS to have portrayal close to Luffy in order for his name to reach the HEAVENS + become the World’s Strongest Swordsman. This does not happen if he simply beats characters far weaker than the ones Luffy faces the entire series until Mihawk comes along. During Yasuie’s speech Oda specifically drew Zoro in the panel where it stated “Kaido’s head has gone unclaimed for the past 20 years”. You figure the hint Oda is giving us.
-Kaido is stated to be UNKILLABLE. Oda has stated Luffy does not kill his enemies at all. Even if he does finish kaido he would Still be a threat to Wano. Zoro is known for his blood lust and having history of killing enemies.
-Oda said Wano is his favorite arc to be drawn. Zoro represents the country of Japan. If his shine does not equal or be greater than Luffy in the Japan based arc of Oda’s country, then all the foreshadowing of Zoro slaying Kaido will be for nothing.
-Sanji defeated Big Mom and saved the Straw Hats during his year at Whole Cake Island by baking a cake to stop Big Mom’s hunger tantrum. He did it by what he represents in the story, and that’s being a chef first, compared to Zoro being strictly a combatant.
*Theory by zorojaan