Originally Posted by AOTKorby
Obito's attached to the Juubi the same way EdoMadara is. He can't use Kamui to dimension shift while that physical connection remains, but if he even temporarily severed it the Juubi would be completely out of both of their control.
Even so, they could just reassert control back over Juubi afterwards. With the shinobi alliance nuked to hell, there would be no distraction for them, so they could put their full focus back towards controlling the beast.