Those a "a bunch of younger guys" are arguably among the top 5-10 defensemen in the league. They are also 2-3 years older than Larsson. All of them had issues with their game in their first season. Larsson's footspeed can be improved.
Also, complaining about Hedman because he hasn't panned out according to the max projection (which in itself is ridiculous)...he's coming around. He had 20 points in 26 games in the KHL, has been above 20 points in all of his three NHL seasons. Guys with his size can take longer to adjust because of their frame.
Hedman is already an incredible defenseman - just because he doesn't put up points doesn't mean that he isn't outstanding. I don't know that I would take him above the 3 defensemen you named but he's almost certainly in the top 30 D men in the league.