He is side character like it or not.
Akainu killed Ace who was trying protect Luffy's life. Akainu put mark on him.
Problem is that Luffy's scar will never dissapear, so you saying he will not care about Akainu case his main goal will be Imu, who did nothing to luffy, while Sabo will kill Akainu? You know that will give zero impact in story. Plus Luffy will nevee avenge even himself
As I said before power lvl scaling doesn't matter if we talk about important for plot fight