There's nothing more unpredictable than a high school female distance runner. It's like trying to predict the weather several months out. Their bodies change and they often have psychological issues (i.e. anorexia, emotional issues, etc - all that is true). Those two things really sum it up. Mary Cain looked phenomenal, then she gained weight and lost her sleek running form. How many others 'phenoms' were anorexic and that anorexia ultimately broke down their bodies? Countless, literally. The stress fractures, lack of food, the psychological burnout; and this disease seems to only affect White girls. Not 100% but pretty close.
As for Sadie Engelhardt, she fits the standard fast freshman/sophomore girl who may or may not run into the female physical and psychological issues mentioned. Zero prediction on her future. You can't project out a girl's running career beyond the next week.