In his Era, Madara is a God. Kaguya is an ancient tale, that's why I dislike the fact that she could become the Final Villain, way too little character development and story surrounding her.
That is a common theme in many Japanese stories, they like to introduce a final villain at the last second.
Kaguya was the one who originally came up with the Infinite Tsukuyomi and started this whole thing, she stole the fruit which resulted in the birth of the Ten Tails.
Saying that Madara is the final villain instead of Kaguya, is the same as having Obito be above Madara. As BZ noted, Madara was just like Obito in the end.
Also like I have said, it would be pointless to have BZ betray Madara and not have Kaguya as the final villain, even Madara didn't see that coming, and why introduce someone that even the Sage fears if she isn't the final villain.
Madara has accomplished his goal of IT, why introduce a whole new concept like Kaguya instead of have this be the final battle, the only conclusion is that their is more going on then Madara knows.