People said the same thing about Obito, and Kaguya is a God, Madara is a man who wants to be a God.
People said Obito wouldn't be TNJ.
Madara wants to become like Kaguya, wanting and succeeding are two very different things. The Sage openly said that their is no one who matches her, not even him.
Even the Sage described Madara as a Indra reincarnation that just got out of line, Kaguya is the ancient being who started it all, she is the entire reason the IT is possible, she is the reason the Ten Tails exists, she gave birth to the Sage of the 6 paths.
Also Kishi has implied in several interview that Madara wasn't the final villain, when asked he got vague and said that people should keep watching, he also said Naruto would handle the rest of the battles like this Madara, Juubi, and Sasuke in that order. Kaguya is clearly part of the Juubi.