How are we supposed to be invested in Luffy vs kaido when kaido is effectively a stepping stone to Teach? Has Kaido actually done anything to Luffy? no, every bad thing he has done has been to the people of wano and the samurai.
Kaido hurt Luffy's friends so we now need 100+ chapters to deal with him. However the only thing Luffy even needs from kaido is his poneglyph. Basically, we are spending 100 chapters and hundreds more in build up, for Luffy to steal a treasure map.
Okay, you can do that. Or you can just focus the story on Luffy finding the maps to raftel. You know, the actual main story. But NOOOOOO. We need 102 chapters of dressrosa, 78 chapters of WCI, 100+ chapters of wano, and you wonder why new fans are hesitant to pick up one piece.