The Fields issue comes down to one thing, do the Bears think his ability to process defense and move through the progression improving.
If not, (or not to the level they think would keep him over a new rookie) they need to look to upgrade. There are two to three QBs the look to be in the top 10 of the draft (and one likely is the 1st pick). If they think there is higher upside, they draft a QB and trade Fields.
If they want more draft picks, they should look to trade down their other 1st round pick.