Imo Kid should be the one to defeat Kaido, and Luffy should be the one to defeat Big Mom, but of course that's not going to happen because we have to have the main protagonist defeat the main antagonist.
Also Oda wants Luffy to be the hero of Wano and not Kid, imagine if Kid actually defeats Kaido and the people of Wano start praising him and even call him the hero of Wano or the man that saved Wano.
Oda wants all of that to happen but with Luffy, he wants Luffy to get all the praise and be called the hero of Wano.