Luffy has beaten Kaido extreme diff, Kaido is slightly stronger than BB and Akainu, Luffy is more than enough to beat him, also when Luffy beat Katakuri, there were talks about him being possibly the 5th emperor and Shanks said "we're close to meeting again, Luffy" and even BB was lowkey interested in Luffy even tho he said "it's still early for you to be called that ZEHAHAHAHA *Darkness grows*" WELL, he clashed with Big Mom, in One Piece Film, Stampede, Douglas Bullet is nearly as strong as Akainu and is a Yonko level but stronger than Big Mom, and Luffy used the strongest attack of Gear 4 with some help of his friends to put him away, Gear 5 would ofc beat Bullet mid-high diff in a 1v1 fight, like with what we've seen with Luffy, that wouldn't enough to fight Akainu or BB?
And Oda would say in an interview "iF aKaInU wAs pRoTaGoNiSt i'D eNd oNe PiEcE iN A yEaR" like he's basing this take after he created the Magma DF 13 years later from creating OP, if he had created Akainu as Protagonist, he'd make him weak first then stronger as the Series goes by, plus back then, he did make Shanks have that mysterious power that scared the Sea King but didn't declare the power name back then and there was still no CoO and CoA, CoO was first time seen with Enel and CoA with Sentomaru, Oda must've been Drunk af when he said that BS.