I still think letting Willson go was the biggest mistake from the last core. Rizzo, Baez, and Bryant all appeared to be heading into decline and/or unable to stay healthy in their last year or two with the Cubs. Schwarber just hadn't shown any real consistency at the MLB level before they let him walk, and the universal DH wasn't in place yet. But Willson was still good (and coming off his best MLB season according to fWAR).