I think it's probable that losing Cole was a significant blow to the team's mojo. He was well-liked by everyone except Sullivan, played well above his pay grade for the most part, and was shipped out for what basically amounts to business reasons during a cup run. I buy that a decision like that could be enough to take people off their A-games.
On the other hand, in truth, the Pens largely lucked into the 2017 cup. They were battered, missing their number one defenseman, and got pushed to the absolute brink of elimination by the Ottawa Senators, a team they should have easily bested. The roster prior to the deadline last season was in no way better than the 2017 cup team, especially at center, and they were not going to get that lucky again. Rutherford had to get a better 3C than Sheahan, and he did. Brassard not working out doesn't mean the trade was flawed in concept.