Short of us landing Karlsson, I don't think we can realistically get another franchise d-man to run the ship. Even with Karlsson, you lose a significant defensive presence in swapping the two. I think Karlsson's on another plane of existence when it comes to transition and offensive output, but again, Letang's true value to us is in his ability to bring a strong defensive presence.
Even if you bring in a Dougie Hamilton or a Jacob Trouba somehow, I don't think those guys are ready quite yet to step into Letang's role immediately. They probably will be sooner than later, especially with Gonchar and Martin working with them, but I'd way rather us somehow land them and give them 2nd pairing responsibility while they grow and develop into that franchise blueliner while being molded by our coaching staff. Give them the Aaron Rodgers treatment where he simmers while Favre plays out the last of his days with the team.
If we can get a guy of that sort of caliber, I'm way less attached to Letang. But that's not a realistic possibility, I don't think. Either you commit to fixing the blueline and getting back to 2015-16 style hockey, which means keeping Letang, or you sell everyone not named Sid, Geno, Jake, McCann and Murray off, and commit to a 3 or so year rebuild. Well, there's a third option I guess, and it's the one I think JR's going to go with, where he stays the course and ensures the slow, agonizing death of this team until we're the Kings--which happens way sooner than most people seem to think.