As seen at the end of the manga, Madara will be joining The kakashi fight as Juubi's Jnchuuriki. If you're wondering why, It's simple.
-Kamui-
There is no counter for kamui, Logically, If madara came to kakashi's fight, Kakashi could easily snipe his head with kamui, but kishi wouldn't let that happen, so he devised another idea on how to nullify kamui.
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That's right, Kamui doesn't work on the juubi's Jinchuuriki. The Juubi isn't able to travel through dimensions meaning kamui is useless against madara/juubito. That's why obito couldn't phase out or go to his dimension as juubito, The juubi is too large, too powerful to be moved through dimensions.
This way, Kishi doesn't have to worry about countering kamui.

-Kamui-
There is no counter for kamui, Logically, If madara came to kakashi's fight, Kakashi could easily snipe his head with kamui, but kishi wouldn't let that happen, so he devised another idea on how to nullify kamui.
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That's right, Kamui doesn't work on the juubi's Jinchuuriki. The Juubi isn't able to travel through dimensions meaning kamui is useless against madara/juubito. That's why obito couldn't phase out or go to his dimension as juubito, The juubi is too large, too powerful to be moved through dimensions.
This way, Kishi doesn't have to worry about countering kamui.