You seem to be under the strange impression that Shanks isn't as much Luffy's enemy as he is his friend. You think he became a yonkou for shits and giggles? No, he wants to become the Pirate King, he wants One Piece.

A showdown is inevitable, the only reason why i'd even remotely agree with you is because Luffy might want a 1 on 1 with Shanks.