His skating will never be the same and now he has to adjust to playing a different game than he did even in 208/19. He can't (or thinks he can't) join the rush as much because he can't get back as quick, and he can't take risks defensively because the foot speed isn't there like it used to (granted he's never been a great defensive skater but was a hell of a lot better than this). It sucks because even tonight, you see a little bit of the old guy with a few rushes up the ice but doubt that he's able to do that consistently anymore. Soon the offense will get going but defensively his best state is definitely on the PK now where mobility is less of an issue where he can focus on blocking shots and winning loose puck battles.
As for whether or not elite Karlsson led teams to success .... in 11-12 they don't make the playoffs without him exploding onto the scene. In 12-13 they only made it because of absurd goaltending after his injury. In 14-15 they don't make that miraculous run without his outstanding play - Hammond was great but Karlsson was the engine of the last 30 games that season. In 15-16 they aren't drafting 11th but 2nd-5th if he wasn't playing at the highest level almost every night. 16-17, well, we know what happened those playoffs, and the season was just as good. 18-19 when the Sharks got rolling in January there were many contributing factors but Erik was most certainly at the forefront of that.
Maybe, just maybe we'll see him play like he did in 18-19, but I doubt he can. He's a competitive guy though, and you can bet he's not happy with his level and wants that to change. Injuries suck and his injuries have come to his greatest asset, and there wasn't even anything he could do about it (Cooke, blocked shot). You have to wonder if he made a deal with the devil as a child to be blessed with the gifts to become an NHL superstar for the cost of a shooting star that burns out too fast.