The "job description" to become Pirate King at the time Punk Hazard happened was "reach the last island and find the One Piece". No where it said: beat the Yonkos. Then out of nowhere we have Luffy telling Law "I was already planning to take down the Emperors. I just don't want to fight Shanks first".
Oda does Luffy a favor by making the Yonko "evil". But imagine if WB was alive. Was Luffy really just gonna find WB and beat him? Some "nice old man that Ace likes"? That's stupid as fuck and only serves to give boners to 12 year olds that want fight fight fight fight.
Anyways, I am planning to make a thread about it in detail....will tag you once I am ready...