Board Thread:Naruto Discussions/@comment-26773508-20150905054531/@comment-5936871-20150905054821

Kurama stated himself that if his other half is sealed into Naruto, Naruto will survive. Kushina and Minato had no way of knowing that unless Kurama told them, which he didn't. So Kushina assumed she would die if Kurama was resealed back into her.