Since prophecies are a thing in One Piece, how about this: Shanks learned years ago Blackbeard will kill him. So in particular, he knows that whoever attempts to slay Teach while Shanks is alive is doomed to fail, but also Shanks himself is powerless to stop him other than through bluffing like Marineford as he will either win or get away. That's why he's so obsessed with him. Shanks is essentially doomed to watch Blackbeard grow in power until he's finally ready to take his life, but instead of trying to fight the inevitable, he spends the time preparing for what happens after while trying to mitigate the damage in the meantime.