It’s never shown what happens to Madaras body at the end of shippuden. I think the anime showed it next to Kakashi and Sakura even after Hagoromo left, but that really means nothing. So say his body is still in their possession, should sasuke try using his old eye? And Naruto can just get a power up by being evil and sealing another Bijuu from 1-7 in himself if he wanted.