The problem with Sullivan is he's become complacent and he refuses to adjust to his roster. A "system" only works if you have the players to properly play it. Sullivan wants the Pens to play like the 2016 Pens, which was a fast paced, in-your-face team that forechecked well and just was faster than anyone else. This year's team isn't that, yet he wants them to do the same stuff.
IMO, the really elite coaches are the ones who adjust their system based on the roster they have to work with. The good but replaceable ones are like Sullivan, who insist their team play one way even if the roster can't. In other words, he's not a "bad" coach. He's just a poor fit for the Penguins now.