It's not being being King of the world this is BB's goal
Also Luffy said it doesn't want to rule over something for him being PK is just being the freest person
Being King of the world or a savior completely goes against Luffy's personality
He also had that dream at 5 because he told it to Shanks.
So also before he Saw all the poverty, slavery, Cds and Kings
It's a dream which made Shanks and Rayleigh remembering Roger
That's why i said i think we must separate what he shout out loud, and what it really mean to others..
He doesn't want to conquer anything & really appreciate freedom, yet he shout out he want to be Pirate King.
While for WG and other pirate, Pirate King is the strongest pirate who conquer & ruled over the sea.
So, Luffy want to be King of the World doesn't simply mean he want to conquer the world, but it's just natural next level of Pirate King / being the most free person in the sea, is to be the most free person in the world..
For 5 y.o to shout out i wanna be the king of the world without knowing what it really mean is just makes sense..
WB reaction to Roger is just on point "what are u? a child?"