I think Thompson can be a 1C on a playoff team, but there'd have to be a really good supporting cast around him. That's clearly not the case in Buffalo. You're on point with Cozens, Buffalo really should have moved him and kept Mittelstadt.
I do think that's a really inaccurate representation of what Dahlin is, He's a hell of a lot more than a "top 4 guy who's a disaster when the other team has the puck". He's a legitimate #1D and frankly he's become pretty decent in his own zone. The terrible in his own zone narrative has stuck with Dahlin from his earlier seasons. Buffalo would consistently be in the running for the worst team in the league without him. Luukonen may not have a long established track record, but he was good for Buffalo last year, he kept them in a lot of games. Honestly I don't see this huge hype train for most of the guys you've highlighted (outside of Dahlin). If anything I think Buffalo's key players have been consistently underrated and we've seen this as these guys go to contenders and flourish.