Let's not forget it's not ALL on the assistants that the power play and penalty kill sucked when it mattered the most--Bednar shares in the blame as well (and yes, it can't be ignored, the spate of injuries took a huge, huge toll on the PK, especially the goalies). He's the one who ultimately decides how the systems that Bennett and Pratt devise are implemented, and whether or not those systems need to be changed.
The power play became effective again when they finally started changing things up and quit relying so damned much on MacKinnon or Makar to zip the perfect shot through traffic. It works best when they can get MacKinnon or Makar different looks and is almost unstoppable when they generate traffic/commotion in front of the net. When they stand still, pass around the perimeter, and keep trying for that perfect one-timer, it almost always fails.
Similarly for the PK, they simply need a more aggressive system that utilizes the speed this team has at their disposal. It'd also help immensely if they have defensemen down low who actually know what to do with the puck. Too often they have guys who simply can't secure the puck or just can't win that battle/race in the corner.
These adjustments shouldn't necessarily require a wholesale systematic change that prompts the team to kick Bennett and/or Pratt to the curb. It just requires those coaches and Bednar to simply analyze what went wrong and take steps on how to fix it.