Then WB would have been called the greatest Pirate (as the title great pirate encompasses individual strength plus accomplishments and crew), but Oda said strongest Pirate. The narrative was very clear calling Whitebeard King of the Seas and the one who rules in front of the throne despite not even trying. With Big Mom putting Whitebeard on a different pedestal by saying "even" Whitebeard and the narrator saying that Whitebeard was the equal of "even" Roger. Sengoku too said that Whitebeard stood above all other pirates.
Also, Whitebeard himself when declining due to poor health, said that he "cannot stay the strongest forever". Not the strongest man or pirate but simply strongest.
Saying strongest pirate doesn't refer to individual strength is odd. Especially since Whitebeard was not the only one with a great crew. Kaido also had a huge army and territorial advantage with Wano. Big Mom also had a huge empire and allies. Shanks has probably the most powerful crew and the strongest affiliations (Elbaf). WB wasn't unique here. Also, it was an old Whitebeard saying that they should not take Kaido lightly. And not taking him lightly doesn't mean admitting inferiority.
So we have this vast amount of portrayal regarding Whitebeard. Why is it significant what an old hag in Wano says. Especially since she would not be able to effectively compare Kaido to Roger and Whitebeard.
Imo, the King of the seas, the one who rules in front of the throne doesnt matter in 1vs1.
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.
Big Mom's "even Whitebeard" proves that it was never about WB being individually strongest since the giant army doesnt help her against WB himself, they help her against Whitebeard's army which is the strongest among all Yonkos.
In pirating achievements WB was never Roger's equal. In terms of 1vs1, he was his superior and when you mix those two it balances out and you could make sense of "equal of even Roger". As I said, depends what Oda means with the term pirate.
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...
Sengoku's fear is logical, even dying old Whitebeard is a bigger threat to Marineford than Kaido could ever be and thus WB is the strongest pirate despite Kaido being stronger in 1vs1 than WB. Thus pirate not referring to individual strength is not odd, imo. Both Kaido and Big Mom put great efforts into strengthening their crew because that's what was needed to take down Whitebeard's crew.
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.
Kaido's advantage is great, yes but that's only thanks to Wano. Kaido going to Marineford would lose all the troops in no time. Aside from him and Calamities, nobody would last long. Pacifistas alone would shred everything from Flying 6 downwards. Meanwhile, Whitebeard's 42 affiliated captains on top of 16 commanders are far better quality.
That's the thing, why is it significant what an old hag says. Why is it significant what King says. Why is it significant what SBS says.
And so on... It surprises me that you dont see how pointless it is narratively for those two to be above Kaido since they are not the ones that faced Joyboy and the main cast... That would be shooting himself in the foot, to glorify characters like that, especially Roger...
It's like saying Madara was strongest without Madara ever returning. Hyping some background character that never faces the main characters is pointless and only dehypes the villains in the present which is not what the author should go for.