Jimmy G has more talent around him in SF than he ever would as a Bears QB. He has one elite WR in Deebo, another really good one in Aiyuk, one of the best tight ends in the league in Kittle (when he's healthy which has been a problem) and the best LT in the league. He also has one of the best defenses in the league (though they're currently being decimated by injuries). Despite all these advantages, he was the anchor that sunk the 49ers in the playoffs. If he were even a league average QB, the 49ers have two championships.
If the goal is to win the Super Bowl, you aren't winning it with him as your QB. There's a reason the 49ers traded all those picks to get his successor.
If Fields is a bust, draft another QB and try again. But for the love of god, don't invest big money in a QB that won't get you any closer to winning a championship.