Actually, you are very much incorrect--prognosis and treatment depends on many factors, including the overall health of the patient, length of illness, environment, whether acquired or inherited. It is most often fatal when the disease goes without diagnosis for some time. Upwards of 90% of patients receiving a bone marrow transplant from a sibling survive AA.
With respect to this treatment however, patients not responding to or are not eligible for transplants are the focus. These people who suffer from chronic, severe AA will be required for trials. Presently, 80% of patients that fall into this category live 18-24 months once they have shown to be non responsive to a transplant.
The time frame that has been established by the doctors and scientists working on this treatment for many years understand better than anyone the variables--especially time sensitivity. That tad bit of importance has certainly been highlighted.