It’s not a coincidence that of the “many legends” about Ryuma, the one Gyukimaru decided to tell Zoro was about him cutting down a dragon above the Flower Capital.
Lo and behold, the final battle of this arc will be... a dragon going to try and destroy the Flower Capital. Where oh where can be found a proud samurai, perhaps one with links to Oden and Ryuma, to save the day?
Whether or not Zoro kills Kaido there or not, Oda’s going to have a scene of him cutting down Kaido in dragon form as he tries to incinerate the capital. With all the natives being there to watch and see someone who’s incredibly likely to be Ryuma’s descendant save them using their other hero Oden’s old sword.
Zoro scarring Kaido, as cool as it was, absolutely did not have a degree of finality about it. No samurai there to see him do it, no real conclusion made about Enma, too early in the fights for everything to be over. Zoro may or may not fight King, that can go either way, I’m not bothered, but King does not provide a full conclusion of Zoro’s plotline this arc, which is blatantly not finished yet. Only Kaido can do that.