Board Thread:Naruto Discussions/@comment-26866422-20160407171305/@comment-734582-20160409122544

Obito could barely snag Kushina away long enough to unseal Kurama. If he had to break the seal first when at full strength, he would've been unable to do this.

If Naruto was captured and the Sharingan used against him, with time the seal would break and Kurama would be free. Or they could simply enter his mental space, remove the seal tag, then force him to give his essence to Kurama. (basically what happened against Pain, but forced to do so by the Sharingan).