There's no guarantee a rookie would be an upgrade
Caleb Williams and Drake Maye, two of the top QB prospects in 2024, are both considered great prospects. However, it's seemingly impossible to predict which QB will hit and which won't. As discussed here, teams have about a 42.9% chance of a first-round QB not working out.
Let's say the Bears draft a quarterback in the first round. If that QB doesn't pan out, you're looking at a few more wasted seasons and being back in the same scenario. Fields has not been elite thus far, but he's been serviceable. In 2023, Fields has 1,976 passing yards with 14 TDs and 8 INTs. The QB has also added 488 yards on the ground and 2 rushing TDs. Could a rookie QB perform better than that? Sure. But it's far from a foregone conclusion that a rookie would be better than Fields.