I think Sanderson got paid based on potential, but I also think he has a very solid chance at living up to the likes of other defenceman getting paid in that range.
This contract on the other hand, is more than almost everyone else. Dahlin basically has to become the best defenceman in the NHL to live up to it. I don't think that's impossible but I think it's less likely than Sanderson being worth 8.
That said, this may have just set the bar for the next wave of top-10 defenceman, so it may not look as high in a few years. Time will tell.