Sanheim isn't as bad as he looked last season, nor is he as good as some here have suggested over the years.
His issue isn't development, it's that his limitations are physical and they're not going to change.
He's a good north south skater once he gets moving, has good but not top offensive skills.
Lacks great vision or a killer shot. He's not a great finisher in the O-zone, better in transition.
Speed more than quickness. and a smooth skater who can carry the puck but not especially agile, not a lot of "shake and bake."
He's decent defensively where he can use his length, but struggles along the boards and wrestling with PFs (high center of gravity).
Sanheim is a 2nd pair offensive defenseman, who can give you 1.00 pp/60 or so, but is limited on the PP b/c he's not great in short areas (squeezed against the blue line as a PP QB). Probably not worth $6M, but as the cap rises the next few years, his contract will become more attractive.