Kaido as a young man believed devil fruits were an essential path to power, a seasoned Kaido believed that you could conquer the world with haki alone and in the end its king. This doesn’t mean after honing one’s haki that you should stop using your devil fruit powers this is what people miss when discussing haki vs devil fruit. Kaido and Luffy’s big final attack wasn’t *just* haki or *just* devil fruit it was a combination. Giant magma dragon form blasting a breath laced in haki and a colossal island sized fist laced in haki and the last word of the clash is Luffy thinking back to his training so that he doesn’t have to make direct contact to strike. The lesson of Wano at least when it comes to battle is that great haki can overcome anything, great fruit plus great haki can be even crazier, and throughout the story we have come to learn that just a strong fruit with nothing else can only take you so far.