Wasn't the point that he couldn't become the PK anymore once Luffy ate the fruit?
That's why he also gave his arm away to save Luffy. He had the potential to become the PK and even more than that but lost it once Luffy came along. Law never lost the chance to be who he's destined to be in the future, unlike Shanks.
Hm, never thought about it that way. Probably due to never believing Shanks ever wanted to eat the DF himself, rather I think he was looking for Ace and by accident found Luffy instead. But it is true that he did lose a few percent strength without his arm, even though I still believe he could have become PK a few years ago if he wanted to.
WB was too old and sick, BM also way past her prime/lower ceiling. BB no DF yet, Mihawk a lonewolf, Luffy too young. Akainu at Marines and Dragon at RA.
Kaido would be his one and only threat for PK with Mihawk/Akainu/Dragon remaining outside threats. And I would still bet on Shanks vs Kaido but I might be wrong, who knows.