In my personal opinion, Kaido is just one of the things that was sacrificed in Wano because of decisions of some high ups that started close to the end of WCI and increased a lot in arc 3.
When we first heard about Kaido he was constantly considered and called a thing, someone that would mask as a human but wasn't so. Instead of prejudice against Onis it looked more like fear. The title of king of beasts and his desire to be able to die made me think of someone unnatural or even cursed. Specially after learning that Zunesha was also someone that was unable to die and had to wander. I even remember someone theorizing that Kaido was one of the ancient weapons. Good times.
But when we actually meet Kaido he ended being just a let down. His fighting style is not the best for long fights and was a little boring after a while, his motivations seemed to jump all around the place, we know that he had a huge territory but the story made it seem like he only had wano, the alliance with BM would shock the world but then he made the BMP get stuck on the waterfall for the whole wano and so on. Overall it got to a point that people started to wait for a flashback to make sense of this craziness. And the flashback ended up being also a letdown.
In a way this is similar to Big Mom. Until the wedding cake she was one of the best designed antagonists of One Piece (she actually earned the nickname of kingpin), to a point that if she had died/disabled because of her hunger pains at the end of WCI and never went to wano the story would be better because of it. Kaido is the same, Oda gave hints that there was some depth on his motivations and actions but when it was revealed to be skin deep it made his shortcomings worse. The payoff wasn't worth the buildup.
I still believe that this was done because of some high ups in the editorial staff and his vacation after the raid ended was him trying to savage what he could to finish wano and start again.