User blog comment:Ghmorello/Chapter 907: Game Of Empty Thrones/@comment-70.122.157.47-20180609035722/@comment-53539-20180609170345

I think they said it best previous, Shanks is not a man who would try and change the world by himself. In other words, he is no threat to them at all. He isn't going to kill or attempt to kill them, he doesn't do that. They don't have to be super strong, they know he isn't going to start anything at all.

I did see someone suggest Shanks is a Dragon himself, but I doubt that since Shanks has been theorised to be a D as well and all that. Fans just want to fill in gaps all the time with him.