Something I took from last chapter, Zoro is actually in the perfect position to slay Kaido over the capital. Zoro is over on the Right Brain of Onigashima, which is the side he cut the horn of the Skull dome. And he is near the edge of the island.
Zoro would notice Kaido heading for the Capital, and would be able to slay him from a view where everyone can see him. Coincidentally enough, the people of Ebisu are almost directly below Zoro. They were the ones he bonded with the most, so it would mean the most for them to see Zoro slay Kaido and free them from 25 years of oppression.
CP0 also isn't aware of Momo, who will prevent the island from falling should Kaido die or be knocked out. Meaning, Onigashima could still inch a little forward so that everyone in the Capital, as well as Ebisu, can see the legends passed down for generations play out right before their eyes.