I've got to disagree with you here. I'll give Cassidy credit for changing the style of play and have this team no longer playing boring hockey. However, I'm putting some blame on him too and the keyword is adjustments. We saw the same two series against Tampa two of the last three years. Not only did they look unprepared to face Tampa, which is on coaching, they lacked mid-series adjustments. They had the same lack of adjustments against St. Louis.
The other piece is lineup flexibility. This has been a one line team ever since Cassidy took over, which makes them incredibly easy to defend. That one line gives them a higher floor, but lower ceiling. I don't care if it makes them a worse regular season team, one of Marchand or Pastrnak should be moved off that line. It makes them more difficult to defend and gives the team a higher ceiling.
Here's a fun fact: over the past three years, Krejci and Pastrnak have better GoalsFor/60, GoalsAgainst/60, GoalsFor%, expectedGoalsFor/60, and expectedGoalsFor% together than apart. The statistics literally say they should play together, but Cassidy refuses to do it for any consistent amount of time. The stats have also shown that basically anyone can play well with the Marchand and Bergeron pairing, except for Jimmy Hayes.
They refuse to change because it "works", but it doesn't actually work. It lowers their ceiling and makes them easier to defend in the playoffs when you can put more focus into stopping certain lines. I think Cassidy is a moron for not changing it despite the fact that it's been their clear fatal flaw for over three years.
Lastly, I'll be honest, I just flat out don't like the guy, which, admittedly, probably clouds my judgement. I don't like how he's always willing to throw a player under the bus, but never takes personal blame for bad coaching. I think that as long as the Neely/Sweeney/Cassidy triumvirate exists, they'll never win a Stanley Cup and I've thought that ever since they were put together. I think we're basically the Boudreau Capitals or the McLellan Sharks and who the f*** wants to be them.