WCS isn't worth chasing, but if he's willing to stay and not demand more than he's worth (which isn't likely since he's constantly talking about money), then let him walk. I can see him in a Noel situation where he declines a reasonable extension in the hopes of a big payday, only to find that the market just isn't there for him.
There are, however, solid big men free agents to consider that can actually rebound that the kings could give that money to:
Those two guys rebound way better despite WCS's athletic abilities. WCS runs well in the space and pace that the Kings play, but if they want to win playoff games, they will require a center that can rebound the basketball and defend the rim.
Guys like Marc Gasol, Al Horford, Whiteside, Deandre Jordan, Kanter, etc are also on the market and there aren't that many teams with cap space. The Kings have a good shot at getting Vucevic or Valanciunas IMO.
Darkhorse candidate to replace WCS is Julius Randle. He's primarily a 4, but he can still rebound well for his size and is tremendously better than WCS on offense. A rotation of Beli/MB3 and Giles/Ju off the bench is insanely talented.
It's nice to have a center who can run, but all five guys don't have to beat their man to the rim in order to have an effective fast break. First, you need to secure the defensive rebound and a Vucevic/Bagley front court would be insane at cleaning the glass.
Vucevic's shooting and passing are ideal for the Kings offense and to complement Bagley. He isn't a defensive monster, but his rim protection numbers are far better than any Kings big man. There's no better free agent target for the Kings.
Now my opinion is also going to be based on how much money it will take to resign Willie. Willie has a certain value, and he deserves to get paid that value, but not over payed. I can see the Kings with a three man rotation in the future made up of Willie, Bagley, and Giles. Start whomever you desire.