Also, if I may, even if Imu really is the absolute last fight of the series, the actual arcs between here and Round 2 really is pretty short. Like, if Burned Guy is at Lodestar, then they have the means for finding Laugh Tale, and then cue the war.
So, really, even if the arcs themselves are long, it's only going to be what...2 or 3 arcs before the rematch. So, Imu being written as overwhelmingly powerful to the point no one could stand against him...only for Luffy to be his equal in like, five days, in story, wouldn't be good writing, either.