I think it highly probable she will not stagnate.
I have not seen an American talent like this since Mary Decker.
Like Mary D, Mary Cain simply has incredible speed, and she likes to race.
And I don't think a good relay leg is a sound comparison to an open 800, particularly one against high level competition. Her 2:01 800 is a demonstrably better performance than her relay leg, evidences significant improvement, and makes me think a 4:05 1500 this year is realistic.
I don't understand the sniping about this young woman leaving her high school team. She obviously has unusual talent, and she and her parents did what they thought was a correct decision in a sport, unlike, for example, basketball, it is necessary to play for a power program to be recruited and develop your game for a chance at the pro ranks. I do hope she has a firm commitment to developing academically as well. It will take the pressure off, and give her another means to express herself - something in my own experience supports being happy and well balanced. I wish her well.
Salazar gets a lot of grief and sniping as well. I think one benefit his athletes obtain is, well, given that he destroyed himself by over-training, he is certainly conscious of the hazards of over-training and the associated injury risk.