Quoting: TCMonkey
You're welcome to think whatever you like, but the return on Karlsson is going to be a 1st rounder, a player and a prospect. There is ZERO guarantee that Ottawa's 2019 pick will be first overall. The odds are actually against it. Very strong chance it's top 10 but who the hell would give up Karlsson for the 6th overall pick in 2019; it's a horrible return.
This is a tough deal to peg because superstar defensemen at the end of their prime aren't traded very often.
If you look at comps, the best I could find is:
28 yo Shattenkirk making $4.5M to WAS: a 1st, conditional 2nd if WAS wins the cup or resigns Shattenkirk, a prospect bottom 6er and a NP; STL also retained 39% salary. the 1st was also known at that time to be out of the lottery. Only half a season of Shattenkirk.
Let's say a full season of Karlsson negates the salary retention and improves the prospect. I would imagine something like Karlsson to Tampa for 1st, 2nd, Cirelli would be closer. Or karlsson to Dallas for 1st, 2nd, Hintz.
Bottom line is no one knows what the return will be, which is probably why a deal hasn't gotten done. I have no doubt Ottawa wants what you propose, I just don't think it's being offered. And for those Ottawa fans adding Bobby Ryan's deal into the mix, the return will be even less.