The difference between Whitebeard and Ace's deaths and "Kaido being killed" is in Oda's interviews on how he feels about death in manga. Oda doesn't kill villains because it does nothing for the overall benefit of the Story. Whitebeard and Ace's death benefited the story as a whole, as does every flashback death he's ever done.
Kaido dying would be nothing but pure shock value and fanservice for a select group of fans. And the shock value would be greatly diminished due to it being what everyone is predicting would happen. Nobody predicted Whitebeard and Ace would die, because at the time it was unheard of, and Oda was able to capitalize off of that, and it worked. It was emotional for a lot of fans, and still is for some. It is something that left a lasting impression, while at the same time made complete sense as to why it had to be done.
There is none of that with Kaido dying, and has the exact opposite effect. A Villain finally gets their dream fulfilled? What the shit is this? What makes Kaido so special compared to all the other villains who came before him, who had their dreams crushed and they had to live with that, knowing they'll never achieve their dreams?
There is nothing being setup here that absolutely requires Kaido dying in order for the manga to have to move forward. It goes against Oda's principles, and mindset that people having to live with their failure, is a fate far worse than death. But nah, lets just go ahead and kill Kaido, because... Because.... Because REASONS!