Headcanon. Shanks just says that it is time to search for One Piece, just after Luffy defeated Kaido. Shanks is still stronger than Luffy so there is no bullshit in saying Shanks can defeat Kaido. Kaido himself admitted that Roger was at the top because of his Haki and that Haki is above everything. Shanks doesn't have a Devil Fruit and his mastery is Haki. Thru Haki alone he is a Yonko which means he will probably have the best Haki affinity mong the Yonko.
The One Piece world turns around Luffy and his actions. Everything happens in the story we read because of Luffy's actions.
The One Piece world turns around Luffy and his actions. Everything happens in the story we read because of Luffy's actions.