Here's the problem with keeping Fields:
When the Giants drafted Eli Manning, they signed a washed Kurt Warner as their bridge starter. This was Warner after he had fallen off with the Rams, but before he had his renaissance with the Cardinals culminating in a loss in SB XLIII. Warner played okay and led that Giants team to a 5-2 start. After dropping two games in a row, at 5-4 but still within playoff contention, the Giants switched to Eli Manning as their starter. Eli lost the next six games, playing well only towards the end of the season, before winning the team's final game in a 6-10 season.
The Bears have put together a pretty good team around whoever starts for them next year. Let's say they keep Fields, and Fields, with the best team he's ever had around him, continues his erratic play with occasional flashes of brilliance, but that's good enough for the Bears to start 6-3. Now the coaches may have seen even further proof that Fields is what he is, and that the team will ultimately be better off, if not immediately, in the near future (2025, 2026, and on) if Caleb Williams started getting playing time. That might be a really hard sell in the locker room, because here's the thing about most of the players in that locker room---they don't give a damn about the 2025/2026/2027...Bears, because few of them have any guarantee that they'll be part of those teams. They only care about the team's prospects in the present season. If you keep a guy like Fields, a guy who was once seen as the team's franchise QB, and you get off to a decent start, you risk creating a locker room of "Justin's guys versus Caleb's guys", and that's not what any coach is looking for. That's highly unlikely to be a concern if the QBs Caleb Williams is competing against are Tyler Bagent and Brett Rypien, as stated above.