Williams' value was highest after 2017. I don't think it was that high after 2018: a defensively suspect player with a 99 OPS+ and checkered minor league history (right or wrong, teams care about that nonsense and talk about that stuff, and you saw it pop up again when he almost didn't report to AAA).
Now, his value was not nothing either. But they decided his value to the major league bench was greater than his trade value. Then Kapler barely used him and he was unable to handle the role. There is something to be said for the older, less talented bench player (such as Gosselin) or the vet who adjusts to not playing every day (Walker).
Really they should have traded him after signing Cutch, even if we theoretically still needed him at that point. I don't even think they would have wanted him in RF, and I think Klentak also failed to consider how much Cutch would play. By the time they signed Harper most teams had their outfielders.
Nick gets some of the blame too. Klentak failed to move him when he still had good value, and the Phillies didn't do the guy any favors, but he took himself from modest value to no value all by himself, with his performance last year, which also prompted the Phillies to get Bruce.