I feel like Oda doesn't want Luffy to have any real rivals that Luffy can fight with everything but Blackbeard. taking kid out of udon was basically for luffy to learn ryuo by himself. ok, after all, luffy is the protagonist, but the way oda did it was very strange. if kid would run away from udon, why not make him run away instead of being caught and used as an incentive for luffy to learn ACOA? does not make sense. with law is like that. in dressrosa we learn that law learn sword techniques and martial arts in the training of the donquixote family, so why not add this to the character's fighting style instead of making him focus exclusively on the akuma no mi. in haki it's the same thing. both law and kid have knowledge and know how to use haki, why not make them use it in a fight. the fight against big mom was the perfect opportunity to develop their haki, with kid learning ACOA and ACOC and law too, after all law is a character that makes sense to have the conqueror's haki