A simple reason why Luffy might Lose again against Kaidou (Unlikely but still possible)
I believe that for Kaidou's Hype as 1 v 1 Champion & Strongest Creature, Luffy's Rounds against him represent the Different Challengers who Kaidou can face in his Life, it also shows what it means to become a Yonko:
Act 1 Luffy = Experienced Supernova Level ---> That Round was to show us how Kaidou easily Defeats Promising New Supernova. (Neg Diff)
However, it was said that Luffy only trained for a Year & Half with Rayleigh, meaning he didn't learn everything from him, hence why we had Udon Training where Luffy learned Advanced CoA Techniques which Rayleigh demonstrated but never taught him. Bringing us to Round 2:
Chapter 1000 Luffy = Fully Trained Supernova ---> This Round showed us Peak Level that a Pirate can reach with only Training but it was still not enough for Kaidou, however it made him get a little more serious at least. (Low Diff)
After that, we started exploring Powers that only Select Few can have, meaning Levels beyond Training, and that was demonstrated by Luffy learning to fight with his CoC which Kaidou described as Power of Very Strongest Individuals.
Chapter 1010 Luffy = Top Supernova Gifted with King's Power ---> This Round (Split into Two) finally made Kaidou get serious & it showed how Rare Power of CoC can boost someone massively beyond just Training, allowing one to go toe to toe with a Yonko, however it was still not enough for Strongest Creature. (Mid to High Diff)
And then Luffy finally reached his Peak Level which represents the Highest Level someone can get in their Journey (Oda even made Luffy say Gear 5 is his Peak) and it basically represents Full Training (Max CoA & Max CoO) + CoC + Powerful DF Awakening ... etc
Indicating that Luffy is finally strong enough to defeat Kaidou, but will it really be enough?
The Three Versions that Kaidou fought before represent the Levels that a Promising Supernova can get to, One with Experience, One with Full Training & One with King's Qualities & even thought the Difficulty kept increasing, Kaidou was always portrayed as Superior & these things are never Enough for him.
So it's in this Fourth Fight that Kaidou gotta finally prove his "Strongest Creature" Epithet & Fame as 1 v 1 Champion, it's in this Fight where Kaidou gotta show how even those who are on Same Level as him will lose against him, he must show that his Ceiling is greater than the Ceiling of a Promising Supernova who had Good Training + CoC + Powerful DF. This is the moment to finally witness Kaidou's Victory Cry