Derrick Williams' best case scenario will be like Antawn Jamison, and while that's pretty good, that's not worth taking over Irving, not by a long shot. Irving can score, facilitate, shoot, defend, and he has good leadership intangibles. Irving is for real and you don't pass up that kind of talent for talents like Williams. Even though the league is flooded with PG's at the moment, I just think that Irving's talent is still far more rare than Williams'. While there may be a ton of PG's right now, there's still a lack of true heady facilitators, and Irving is that. I may just be geeking out on him, but I think what I'm saying has merit.
Also, I really don't trust Evans to run an offense. Taking Williams is putting way too responsibility on Evans to incorporate the other scorers of Thornton, Cousins, and Williams. I don't he's ready to do that, and honestly, I don't he'll ever be up to that task.
Right now my top 5 list for the Kings is: Irving, Valanciunas, Williams, Sullinger, and T. Jones