Why is Madara supposedly dead? According to Kurama, the Juubi's Jinchuuriki Wouldn't die when the Bijuu are extracted from him thanks to the Gedou Mazou. 



So how could Madara have passed away?
That's because the Gedo Mazou itself was extracted as well (and sealed on the new moon on the other dimension).

Spoiler


   
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 We can see the Gedo Mazo getting caught up in the Rikudo Chibaku Tensei, while Madara getting spit out.


Madara didn't have the Gedo Mazou in him to provide life force when all of the Bijuu were extracted.