Everyone knows he sucks defensively, the disagreement is that people don't think his horrible defense outweighs his offense and puck moving.
Karlsson is the 12th highest scoring defensemen in the NHL this year despite being on an absolute mess of a team. Even with how he has declined, he's still one of the premier OFD in the league and brings a skillset that not many other players in the NHL can offer.
You bring up Tyson Barrie and the comparison is appropriate. Barrie was a very useful defenseman until he stopped producing. He only fell out of usefulness in the NHL because he went from a 55-60 point guy to a 30 point guy, at which point his defensive issues overtook the offense. The same thing happened with guys like Schultz and Yandle as well.