Two issues with this.
1. Imu was not an asspull. We have known him for 100s of chapters already. And the world government, like gorosei, cpo and cd, were antagonists since part 1. Kaguya was first mentioned by Edo Madara without any background. Then suddenly kills Madara and became final villain.
2. Imu is not final villain. The real final villain is 20 years in making since the jaya arc. Imu is more of the climax of the story against the wg/and one piece treasure.