Thornton is not a 3rd line center. Why would you voluntarily cut your ice time, take less years and less money? If the Kings sign Thornton it would be to play #2C and move Carter to the top line. Thornton averaged 18:04 of ES ice time and a whopping 3:31 of PP ice time where he put up 19 points. To compare ice time for the uninformed saying Joe would be a 3C Kopitar played 20:46 at ES and 3:00 on the PP and also put up 19 PP points.
Thornton just trashed his knee, is a year older, and he doesn't exactly strike me as the "win first" kind if guy. I'm not saying it's likely he comes here, but it's not completely impossible.