ZKK make sense because of Kaido's character.
Do some except Kaido to spend the rest of his life rolled up in a ball and crying like a little bitch just because his 8th defeat came from Joyboy (Luffy)?
Kaido should consider Joyboy just another opponent that couldn't kill him and continue on his journey. Kaido gauges opponent's worth in battle. Even though Joyboy (Luffy) is a worthy opponent that doesn't change that fact he couldn't kill Kaido. Kaido has lost to 7 other opponents as a pirate. None of those opponents were able to kill Kaido so Joyboy should be added to the list of other worthy opponent who couldn't kill Kaido.
Oda set Kaido up from the beginning as a villain (obstacle) that must be killed (removed) to overcome. Just beating him in battle won't be enough even if the defeat comes from Joyboy.
Luffy is powerful but doesn't have right tools for the job. Luffy knows he can't become PK alone. That why at the beginning of his journey he set out and recruited Zoro and others. It's time for Luffy's fan base and others understand that.
Zoro has the correct tools (skills/power) to kill (remove) Kaido. Zoro is a swordsman who possess incredibly powerful cutting attacks. The only way to kill Kaido is to hit him with a single attack powerful enough to completely cut through his body with one hit. Zoro fighting and beating King helped push him closer to that level. That the reason Oda gave King greater durability and defense than Kaido. By having Zoro face an opponent harder to damage than Kaido it forced Zoro to take his skills and cutting power to a higher level.
All that is left is for Zoro is to forge a black blade. Forging a black blade will increase Zoro's offensive power, defensive power and CoC.