I don't think Shanks is that much stronger than Kaido (if we assume he is and he probably is if he appeared in that vision of how high is Luffy's ceiling scene) the "vibe" we get as readers is Shanks has achieved the pinnacle of Haki mastery. Fruitless, 1 armed emperor of the sea. Draws a sword and ends war because no one wants to fuck with him and his crew.
If he confronted Kaido the reason why Kaido probably backed down is not because Kaido couldn't take Shanks on 1v1, but because Shanks' crew members are monsters. They would eat his crew alive. Highest average crew bounty is now canon.
If he confronted Kaido the reason why Kaido probably backed down is not because Kaido couldn't take Shanks on 1v1, but because Shanks' crew members are monsters. They would eat his crew alive. Highest average crew bounty is now canon.