Jim Ryun qualified for the Olympics after his junior year in high school. However, he faltered in the semi-finals. Compare that to Cain, who qualified for World Championships after her junior year in high school. She has made the final. At this point in her career, compared to Ryan, she now has a greater legacy.
Now, Cain's legacy may not be greater than Ryun's once she leaves high school. She needs to break the women's US 1500m record which is 3:57. If she breaks that next year, she will still be on pace to have a greater legacy than Ryun. If she medals in two days, that will also keep her on pace to have a greater legacy than Ryun, even if she doesn't break the 1500 record while in HS.
For Cain to officially solidify herself as the greatest young phenom in US T+F history, she needs to get gold in 2016. If she comes away with silver, there's still a chance that her legacy is greater than Ryun's.