I think Shanks'll live, but Luffy's coming to defend him, no questions asked.
Here's my theory of the Yonkou:
Luffy/Law Alliance teams up with Big Mom -> They fight Kaido, Big Mom & Kidou's armies are depleted
Kidd Alliance goes after Shanks, Luffy steps in, but Shanks's group is depleted.
Blackbeard, now having no significant competition from any one Yonkou, takes advantage of the situation and, having harvested the devil fruits to power his army, he makes a move on One Piece.
Of course, this can have multiple variations, it's 100% speculation, but I think the overall goal here is still that all of the Yonkou will be out of commission and relatively unable to stop Blackbeard from going after One Piece. We already know that Blackbeard is patient, cunning, and opportunist. In every situation, we've seen him play other groups against each other in order to take advantage of the chaos. I also think that one of the reasons none of the Yonkou are constantly going after One Piece is because they know the other Yonkou would make a move.