Klingberg himself is still good, but the things around him that boosted his value 2-3 years ago aren't as good. Seguin's had a tough-luck year. Benn is in the decline phase. Radulov turned into a pumpkin. There just isn't as much scoring talent in Dallas as there used to be. Heiskanen is the better all-around defenseman, but that helps Klingberg, as the Stars probably would not want to overload Miro with first pairing, PK and PP time. Especially when Miro probably isn't appreciably better than Klingberg on the power play.
Based on the information provided, Klingberg, as a certifiable PP1 defenseman, is probably a keeper for your league. There aren't many guys to whom you can point and say "he's almost certainly going to be a PP1 defenseman." Whether he's a keeper for you depends on your roster.