Roger, Whitebeard, Oden all died for something greater than themselves and what made them truly great was their unwavering belief and spirit in it. Over the course of this fight Kaido slowly seems to starts to see something similar in Luffy and rejoices.
As I said above Kaido is completely empty and seems to not really believe in anything yet his ultimate will is to die like Roger, Whitebeard, Oden leaving behind something about them that in a way immortalises them more than their strong bodies could yet his ways are superficial.
And as a result Kaido will not be able to start a great random war, he will not be able to stop 800 years of prophecy of Joy Boy coming to Wano nor will he be able to die a great death like Oden the way he is.
So I think Luffy eventually defeating Kaido will ultimately mean, the spirit of a king vs the strongest body.
That though Kaido is the strongest creature on Land, Air and Sea, he falls short of Luffy’s will in Land, Air or Sea, that this is the area where Luffy will surpass Kaido by the end of the arc.
*Theory by rx-ne
