There's 0% chance Luffy defeats Kaido on his own. One of the main themes of the arc is that you should ask others for help for things you can't do on your own. Oden should have asked the scabbards, Sanji asked Robin, Momo asking Luffy for help against Kaido...
Oda already showcased how necessary Sanji's cooking skills were against BM. It is very clear that he will show how important Zoro's strength is against Kaido as the fighter of the crew. Kaido the emperor obsessed with individual strength and BM the gluttonous emperor. And before anyone says he already displayed his strength on rooftop, it wasn't good enough. Zoro himself admits that his strongest attack didn't have much effect against Kaido. Imagine in WCI Sanji stops and says that his cake isn't tasty enough to satisfy BM, impossible
Zoro is unfortunate in that he is good at the one thing Luffy and the rest are good at which is fighting. To display his importance Oda gives us scenes that shows us that he can't be replaced like the scene in thriller Park. Every crewmembers is ready to sacrifice himself for Luffy but only Zoro can survive it because he is Luffy's shield. And one of the most underappreciated aspects is that Oda usually does that by comparing home to Sanji the second strongest Strawhat. Sanji was knocked off by a slight hit from Zoro while Zoro takes all of Luffy's pain and still standing up. The way Oda is writing Onogashim:
1- Let Zoro fight against Kaido to prepare your readers for the eventual Z&L vs Kaido
2- Convince them that Luffy will beat Kaido 1vs1 and preserve monster trio dynamic by having Z&S fight together
3- Send them to the rooftop to help Luffy against Kaido
4- Strawhats beside Luffy and Zoro collapse. Luffy does the most damage but Zoro helps Luffy as well
You people have no faith or patience