Even if you dont believe that Kaido was strongest back then, he is the strongest now and isnt King of the seas nor is he ruling in front of the throne. That's exactly what shows why WB was the strongest pirate while Kaido is far from the strongest pirate. You could argue that he is the weakest pirate among the great pirates.
Narratively stronger and stronger people appear later. First one to appear was Mihawk and was World's Strongest. Then Whitebeard appeared and he took over as World's Strongest, even tho he actually wasnt due to his condition. And latest one to appear was Kaido and he took over as World's Strongest who has been that the whole time, ever since Kurozumi Higurashi named him Embodiment of Strength, in the era of prime Roger and prime WB...
Sure, WB's words that he cant stay strongest forever work, because Kaido isnt a thing at that point. Same thing with Mihawk being called the strongest in first 50 chapters when Whitebeard isnt a thing... It is just a narrative attempt at hype. You meet 3 guys who are World's Strongest while actually only one among them is but it is fine because at the time of their hype you didnt know that the other guy was actually the real strongest.
Well, there is a problem with this. First of all, Whitebeard was not someone to actively surpass for Luffy. He died during Marineford being eaten up by his own disease. Secondly, Kaido isn't the final villain, maybe not even the second last, as we still have Blackbeard and Im. Even if we treat Im as a wild card (though he is set up to be the anti Joyboy), we have Blackbeard. You shouldn't make the same mistake as those saying Shanks > Mihawk purely because Luffy wants to beat him, since Shanks isn't the final villain either. Roger and Whitebeard were the peak of the old era, which the new era will surpass through Luffy and Blackbeard. Kaido was just another silver medalist who got beaten just right after Luffy awakened his fruit, yet not being at his peak just yet. Whitebeard being introduced didn't disprove Mihawk's WSS title. While Whitebeard did not get dethroned either, as we established.
Power wise, it would rather be Roger >= Whitebeard. They were portrayed as equal fighters, while Roger had like a year or so to live, with severe illness himself. And Kaido specifically saying how Roger conquered the world with power (Haki) contradicts the notion of Roger being just held in high regard due to his group achievements. Like literally, Sengoku said that Ace had the potential to stand atop of the world merely by having Roger's blood running through his veins. And that is even though Ace rejected being associated with Roger at all.
