luffy defeats the main villain (most of the time) and then the main villain is left alive..zoro has never killed a main villain after luffy defeats one...why would kaido be different ? thats the flaw in "i want luffy to defeat kaido but zoro to kill him afterwards"
And we are moving on to the last arc, having another Villian with that big of an impact, will be extremely hard for Oda to follow
And if he doesn't die, then he will participate in the great war, which is like, his 2nd other dream, "of wanting to start the biggest war the world has ever seen"
I'm sure others can fill it for me.