Fox is one of the best defenders in his own zone in the entire league. Among the elite D, he's one of, if not THE most balanced in terms of elite offense and elite defense. I can show you video clips where he single handedly breaks up multiple 3 on 1 rushes. Don't judge him based on one bad playoffs where he got his knee collided into early.
As for Zib, he's definitely reached his ceiling. But his ceiling is very good. He is a 35-40 goal, 75-90 pt Center who is a master snipe on the PP and excellent at killing penalties. During the COVID shortened 2019-20, if it was a full season he was on pace for nearly 60 goals lol. I think sometimes people actually UNDERRATE Zibanejad because he's better than the guys like Necas, Barzal, etc, so we expect more, but he's not quite on the level of the big boys in the Top 10, and people say "You can't win with him". He's an excellent player.