Who adds what?
Joined: Jul. 2017
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Niskanen was top twenty among defensemen in even strength points again last year, but is not top 20 among defensemen in salary. You're right that Merrill doesn't bring much to the Caps, but Nosek would have a tough time getting a roster spot from Travis Boyd.
If Vegas wants Niskanen, they do have a few defensmen they could almost trade for him, but they'd have to overpay to get Niskanen's toughness, penalty killing, shot blocking, and leadership: McNabb and a 1st, Miller and a 2nd, or Theodore and a 3rd would make trading Niskanen sound reasonable. Otherwise, since he's got as many years left on his contract as Ovechkin, he probably plays it out and those two deals define the Caps' window to keep contending. None of the free agents left on this year's market would be a better use of 5.5M than Matt Niskanen. No one better than him in next summer's UFA crop will sign below 10M. In a salary cap league, you don't give up on a #1 defenseman at a #2/3 defenseman's salary, unless you're getting back a younger, cheaper, likely future #1 defenseman, or at least a solid #4 and a 1st pick.
Even with the above overpayments, since Todd Reirden's first request on joining the Caps was that they sign Matt Niskanen, there's a decent chance that the deal still doesn't get done.