The debate with Mac seems to be whether he is a decent QB who was ruined by coaching and stuck in a bad situation, or is he just a bad QB. I have gone full arc on this. After his rookie season, I was hopeful, after last season, I am not. Maybe he can figure it out, but I don't think it is likely and I don't think the rest of the NFL sees much value in him at this point.
As to Fields, I think he also had a difficult situation, better maybe than the Pats the last two seasons, but if better, not much better. I don't think the OL was good and I don't think the coaching was good in CHI. And yet he was able to perform at least at the minimum level you expect of a NFL QB. 16 TDs, 9 Int is much better than Mac at 10 TD and 12 int. I didn't see him play much but going back to the last game vs. the Pats, he looked decent.
I guess no one wants a fringe starting QB these days, which is what I think Fields is at worst. It turns out that most teams felt a 6th round pick was more value than a fringe starting QB on a rookie deal. I admit, I am surprised. Mac has been more like a fringe back up QB the last two seasons so I am less surprised by his value.