Based on the provided options and the context of the Gold Rush, the most accurate inference about its impact on the city of San Francisco would be:
**The mass migration of forty-niners transformed San Francisco into an economic hub practically overnight.**
This response suggests that the influx of people during the Gold Rush significantly contributed to the city’s rapid growth and transformation into a major economic center. The other options either downplay the Gold Rush's impact or suggest behaviors that contradict the known history of many forty-niners staying in San Francisco after their initial attempts at mining.