I agree with you about the weight thing. However, I do think that this transition will take some time. Its easier and safer to drop the weight once the body is done adjusting to all the age related changes. Many current elite women were once heavier around this age - Simpson, Rowbury, Hasay, etc.
Many young women who go through this maturation stage don't ever make it to the next level. However, many people don't make it to the next level PERIOD. In Cain's case, I think she has the potential to be fine because she has already been at the elite level, and thus knows what it takes and has the confidence that she belongs there. (As opposed to an 8th grade or HS phenom, who dreams of getting there someday but experiences this setback and eventually fades).
There are a number of reasons that phenomenal young talents don't ever live up to their promise. Some of them are physical, but I would argue that a lot of them are also mental and environmental. To argue that Cain is done because many young girls who have traveled in a similar path before her never amounted to elite athletes is ridiculous. This type of setback was inevitable. It doesn't prove that she is "done" just because it is happening now. Where were all the critics when she was running well? Did everyone really think she was going to continue on that trajectory forever without any maturation, stagnancy or setback?
The odds are against any young runner to make it to the elite or olympic level. However, Mary Cain has the advantage of already knowing she belongs there. Give her a year to work it out.