The Rantanen situation closely mirrors what happened with Huberdeau. Think about Huberdeau at the time of the trade. He was a franchise legend in Florida, leading them all-time in points, assists, and games played. He posted several impressive seasons, including his standout 115-point campaign just before being dealt. Upon arriving in Calgary, he was clearly upset by the move. In his own words: "Emotionally, it's been a roller coaster for the past few weeks. Obviously shock with the trade. I was a little down [...]"
The following season, without the benefit of playing alongside a top-5 NHL center and contending team supporting cast, his offensive production plummeted. Now, Calgary is paying first-line money for a player delivering second line production. Huberdeau is also on the wrong side of 30 and under contract until he's 37 (through 2031), which suggests the situation there is likely to get worse rather than better.
The key difference with Rantanen, unlike Huberdeau, is we're going to get a decent sample size of what he looks like without playing for a top-5 NHL center, and so far, the results aren't pointing to a 50-goal, 100-point player. Montreal's supporting cast is stronger than what Huberdeau has in Calgary, but by how much? This forum would be in meltdown mode if the Habs signed Rantanen for $14 million and he delivered at an 80-point pace.
There's no doubt he was an excellent piece playing along MacKinnon and Makar, but I suspect what we're seeing now is a more accurate representation of his abilities on a team where he's expected to be the best player, and so far, the player we're seeing is not worth $14 million. Colorado recognized this as well.