Luffy declares he's going to beat the Yonko's on multiple occasions. Declares he's going to beat Kaido. Declares he's going to beat Big Mom. Flat out says he can't become King of the Pirates unless he defeats these individuals. However easily you think it is to become Pirate King, Luffy (Oda) does not agree with you.
Luffy said hed beat each and every yonko and admiral he will do neither and still become king
I go by the manga facts of someone becoming pirate king,
Roger became pk because he begged his rival for his crew mate stole a copy from meme and had every thing else fall into his lap beating no one on the way. The panels below show very clearly there was no strength involved in becoming pirate king. Its just a journey anyone with enough luck can do it.
And the second pirate has been handed 2 rps without him doing anything for them.