I wouldn't say the Canes would get worse, Dermott has been pretty steady for his age. 22 and in his first full season. This deal would be a no go just for the simple fact swapping a D for a D wouldn't help both teams.
I would disagree.
1) He'd be bottom pairing on Carolina behind Slavin and CDH. Strengthening the bottom pairing at the expense of a top 4 D makes the team worse.
2) I like Dermott, but he still hasn't faced the level of competition that Pesce has and shown he can handle that load like Pesce has shown for multiple years. Pesce, right now, is a better player. That may change down the road as Dermott is 2 years younger and I think he has more offensive potential than Pesce, but right now, that's not the case.
I agree though, that it makes no sense swapping D for D. From a Canes POV, they need a forward back if they are trading one of their RHD and this doesn't accomplish that. Toronto fills one hole and creates 1 hole (particularly when Gardiner is gone next year). They also need Dermott's low cap hit right now.