Most linkshell leaders I've known that have disappeared have literally.. disappeared. So I don't think they had the time to pass leader status to someone else before they did.
Wouldn't it be better if they had something higher than linkshell sack, but lower than leader? Like, where you can promote other people to sackholders, but not break the whole linkshell. I remember in City of Heroes, if you were offline for more than a month, you got demoted to normal member, and the person who was last online was the leader. It got kind of annoying if you just wanted to take a break from the game, though.
I'd be a little paranoid if they had pass-the-leader, since I've heard a lot of stories about leaders getting into relationships with random new members, I bet there'd be at least once incident where a leader would pass the status to their partner, and TERRIBLE THINGS WOULD HAPPEN!