I know you hate the idea of Imu being the final villain
@SakazOuki but be prepared for this plot because it is very highly likely at this point.
Now, will Luffy vs Akainu not happen? It will happen - but before Luffy fights Imu. Or like
@Fleet Admiral Lee Hung said: Everyone vs Imu. If Skypiea is like the final war but on a smaller scale, then it's likely that everyone has to gather against the current "god" of the world just like everyone fought Enel. Also, there are theories that Imu even commands the huge world serpent (some theorists even theorized how this world serpent ate several moons around the planet), so it'd be even more important if all characters would jump on Imu. Maybe Luffy and BB will take on Imu while everyone else have to deal with the world serpent... or it is really everyone vs Imu in which only Luffy and BB remains at the end.
This chapter is just a small demonstration of Imu's power. Did Mihawk destroy an island in his introduction? Or Kaido? Or Akainu? Or Whitebeard? And by "destroying" I mean wiping the island out of existence. That's completely annihilation. Even if Imu used a weapon or is a weapon, it's clear that he holds huge amount of power. And no, I don't think he's just the "most powerful Spamdam" who just needs to push onto a button...