Bridgewater won't improve them. Smith won't improve them. Newton won't. Dalton won't. Neither will Fitzgerald. There simply isn't one on the market.
Can they win with Jimmy G.? Sure. But the Niners know that he isn't what they need to be an elite team. His limitations have been discovered (as they were for Goff) and so D's make him less effective.
I guess I'd like to point out that despite the "no" votes from posters on the Rams acquiring Stafford, he very likely DOES make the Rams better. He was the only QB available of that level. All others are a catchall of lesser NFL talents that either never panned out as hoped for, or have had their game depleted by injury or age. And whoever takes them will pitch that a new surroundings will help them reclaim this or that QB from mediocrity.