If you view it without the revenge element, we basically guaranteed ourselves a "3C floor" player from our 27th overall pick and paid a late 3rd to do so. Add in the upside, well... I'd do it again in a heartbeat, and Kotkaniemi never needs to become a consistent 50 point player for me to continue to say that into the future. There were a lot of peripherals involved in it as you mentioned (the revenge element, smug Habs fans not knowing how weaponizing cap space works, etc.) that sort of clouded the acquisition. But we'd be absolutely thrilled if we drafted him at 27 overall as a 21-year-old overager. And at the end of the day that's basically the price we paid for him.
This all makes perfect sense. There's another aspect to this though. For a team like Carolina that is a cup contender, the value of a 1st and 3rd round pick isn't necessarily the guy you might draft with them, it's the acquisition opportunity they allow. The question for me is "was that the best use of those assets"? I didn't feel it helped fill a big need, at least short term, but we'll never know for sure, so it's speculation. It also prevented the team from making any meaningful deals at the deadline, both because of the cap hit and not having a 1st round pick.
Tampa Bay used their picks for deadline deals/trades for guys with a year or two on the deals (Coleman, Goodrow, etc..) to give them an element they didn't have and push them over the top. I felt Carolina was in a similar situation last year (and this year) and maybe could have made better use of those assets.
That's why I was lukewarm on the offer sheet from the start. I had no issue losing the picks for Kotkaniemi (in terms of losing out on draftees), because as you said, that, in a vacuum, is worth doing.
Kotkaniemi's salary is commensurate with a 3C, particularly as the cap rises so if he plateaus as that, it's still a good deal. If he becomes more than that, it's a bargain. I think the team needs him to be more than that this year for them to be a contender. It's early and the line is playing awesome, so it's not a big concern right now.