I just don't understand this take.
It sounds like you're saying, "They had to find a way to keep Rantanen, no matter what it cost. Everything else makes you the dumbest ever."
But that's no basis for such an absolutest statement in a salary cap era.
When you've already got several players making between 9-13 million, finding a way to keep another guy docked to earn 12-14 million for 6+ years is about the most improbable path you can take.
It's gotta be safest to go on the basis that if it would have made any economical sense for that franchise moving forward, they'd have kept Rantanen in the fold.
If it doesn't, YOU HAVE to move him, even if you suffer in the short-term knowing that he gave a far better chance to be successful in the playoffs.
The actual trade to Carolina was a pretty doggone good one having made the decision that you won't be able to pay him.