My take as a Packers fan, having watched many McCarthy-coached seasons, is that he would absolutely be good for Caleb Williams. He was a "QB whisperer" early in his coaching career. He helped Favre at the end of his career, coached up Rodgers (look at his early mechanics), and has done well with Dak.
I know Bears fans don't want an older coach and/or retread, but he would bring stability, experience, and winning to the Bears. And I think he'd be a great fit for Williams.