There is basically no circumstance that made sense to keep him, and clearly the Bears agreed with that line of thinking as they were only able to obtain a 6th for him.
The Bears have never developed a QB, as we all know. They are to blame for that, but it’s also incredibly difficult to do, as many other teams around the league also know. They are investing the largest draft capital possible in Caleb, they better do everything in their power and give him every possible chance to develop and succeed. Having Fields on this roster hinders that in multiple ways, and clearly they decided he wasn’t going to be part of their long term plans. Once they made that determination, getting him off the roster was the only choice they had, and I believe other GM’s knew that, which is why they accepted the best offer on the table, which was a 6th.