While he became a great player, at the time he was drafted (54th overall) Duncan Keith was hardly considered to be a high end d-man. There were also quite a few other d taken before Karlsson in 2008, and while he had a high ceiling there are 12 other teams who probably wish they had taken him instead of who they picked (pretty sure Tampa and LA were happy with their picks). You are right though we do need top end D, it's just that this scouting and drafting thing isn't as easy as it looks.