Board Thread:Naruto Discussions/@comment-1018594-20160409062530/@comment-25122992-20160815161159

I don't know which interview it was, but I surely heart that Kishimoto stated he didn't know how to defeat Madara (after the Rinne Rebirth). So it's pretty logical to conclude that Kaguya, a more defeatable villain, was put on the stage instead of him.