I think people here are really under-estimating what a top-4 defenseman with a friendly contract can fetch.
Two summers ago, the Bruins traded Dougie Hamilton to the Flames for the #15 , #45 , and #52. Now he's an annual cap hit of $5.75 until 2021. Muzzin is signed for only $4M cap through 2020. Yeah, you're giving up 4 years but we are also talking about defenseman years. Not a big deal IMO. The point is a cost-controlled top-4 D should fetch a ransom in prospects based upon recent history.
Also, there was a rumored Lindholm for Lande rumor before the Ducks finally re-signed Hampus. Another very promising young defenseman (only 22) but was a straight up deal for a guy who carries a 5.25M cap hit. I look at the Kings recent deals like Lucic and it seems like teams are willing to pay a premium for retained salary. Eat a couple million so that a contender can go over the salary cap and you can get a nice draft pick thrown in. The AAV savings on Muzzin's deal, IMO, is equivalent to the age difference of a younger guy like Hamilton or Lindholm.
If the Kings move Muzzin, they should try to get a killing in return. He's probably the most valuable trade asset on the roster (since Doughty is untouchable).