They want a D the same age, and they're not getting it. No one is trading a left hand version of Trouba, because teams don't trade players like that unless they're forced to. From there WPG is looking at getting a decent top4 replacement and a high end prospect like Chychrun or getting a good top4D not in the same age range.
As far as the cap goes, the cost of Trouba would at minimum have Muzzin or Martinez going back. So you're looking at a +2M differential on the cap. That can easily be made up a number of different ways.
It is about what Lombardi wants, but what is the solution on the second pair RHD? Martinez is a fine stopgap.
LaDue? How far out is he and how long until he's playing like a top 4D? After LaDue is there anyone else on the team to fill that position?
UFA's next year, there's Burns. Personally I don't think that realistic.
At this point Liljegren is about as realistic a solution as the others.
http://www.eliteprospects.com/player.php?player=224910
I'm not seeing a good solution for that RHD spot.
Now on the left hand side the Kings have Muzzin/Martinez/McNabb/Forbort/Gravel. It makes sense to trade someone from the left to bring in someone on the right. The team is better off sorting that out. Also grabbing someone Trouba's age helps the team get younger. Since we now know it's going to be a year or 2, at least, before they're back fighting for another cup.