but that's not how narrative and plot go
Lets talk plot, motherfucker.
Luffy has two equals in the road to PK: Law and Kidd.
They're each taking on Luffy's ultimate antagonist for the PK and Luffy's mentor.
Luffy's ultimate antagonist, in order to become an absolute monster ansdgive Luffy the fight of his life, the ONLY fight capable of granting the PK status, will need to also kill the mentor.
The antagonist will already be one step above Luffy by defeating Luffy's equal (Law).
But he will only be above Luffy, forcing the underdog position Oda so loves and which all good combat storylines thrive on, by showing that he can also trash someone else already above Luffy. How do you show properly that the mentor is above Luffy? By having him defeat another Luffy equal: Kidd.
It creates a pecking order: BB>Shanks>Kidd/Law/Luffy. A pecking order Luffy will triumph over, cementing himself as the one and only PK.
Im not saying Law or Kidd need to die. Oda would never do that. But their cockiness needs to be crushed once more, as when they thought life in the New World was easy then they met Kaido.
But BB and Shanks, narratively, since you wanted to bring narrative theory in the discourse, well, BB and Shanks can only win. For BB to later kill an already defeated Shanks will not deliver the full dramatic blow. And for Kidd to also rise to the Yonko title on his own, while Luffy needed help and while Law gets defeated by BB, also creates a narrative imbalance with no solution to the Raftel narrative.
Luffy, BB, and Shanks are the only three actors that will fight for the PK.
The rest are actors necessary for the postPK End War against Imu.