Kaido also said that Luffy couldn't become Joy Boy and guess what happened.
Kaido also believed that Joy Boy will be the one to defeat him and we know Luffy is 100% Joy Boy so Kaido has to be wrong on Joy Boy being the one to defeat him. Not only that, but people who think Kaido died would also mean Kaidou got his wish of dying which also contradicts that idea if Kaido were to die now