1.he did cut Kaido
2.Orochi is main reason for Yasuie death not Kaido
3.Enma becoming black sword, that can happen later in the story
All cap. Zoro literally gains nothing fighting king in terms of improvement of his swordsmanship
King isn't durable like kaido.
Zoro already knows how to cut fire
Kings speed and flight isn't something new that Zoro hasn't fought against. He has already fought inviduals with incredible speed and flight. That's why he learned air slashes
You think oda would introduce Enna as a blade that can cut kaido for Zoro to use it on king? That's stupid.
To add oda even provided another way in which Zoro can turn another besides Enma into a black blade this arc. And that blade is wado. But having a sword creator of the kitetsu state that the same person that made Enma also made wado which was the reason why Momo's sister felt the need to trust Zoro with the blade in the first place.
Kaido as shown as of right now (besides maybe Yamato) is the only one who has shown coC and was the first person to point out zoros coC. Who better for Zoro to increase his coC than fighting kaido?