Where does Lowry fit in the top six? Between Kopitar, Carter, Toffoli, Pearson, Brodzinski, Gaborik, Kempe, etc there is no room for someone who produces at Lowry's pace. He had 29 points in 82 games last year, that is not top-6 level production.
"Grit" isn't what lost the Kings games last year, lack of consistent goal scoring outside of two players is what cost them. If they are going to trade a player of Muzzin's caliber on his contract even after his bad season, then it needs to be for someone who will fill an actual need and make a difference on the score sheet.
There are other ways to add physical play to the team, you don't trade your second best defenseman on an already young defense for "grit" especially when the team badly needs another offensive difference maker.