Imu is the shadow of the past, a relic of 800 years ago. Teach is the real threat of the present. Even Shanks fears him more than anyone else.
Having Luffy defeat someone like Imu at the end, whom he’s not even aware of for most of the story, just by following the path of destiny, would honestly feel underwhelming.
Why did Oda go out of his way to introduce Xebec? Why give Teach such a deep and mysterious background, tied directly to that legacy, if not to have him surpass it?
Roger and Xebec both failed to reach their goals. Luffy and Teach are their successors, and the story is building toward them finishing what the previous generation couldn’t.
Teach is Oda’s most carefully crafted antagonist. It would make zero sense to waste all of that setup just to have him go down before the finale.
https://worstgen.alwaysdata.net/for...ffy’s-true-final-opponent-in-one-piece.68126/