Why would a person with a fibrillating heart be unlikely to survive for long if not treated?

Ventricular Fibril'n. Ventricular fibrillation will result in irreversible brain damage. However if CPR is initiated early, it may increase the chance for survival. Also hypothermia (a lower body temperature) may also increase the chance for survival.
Ventricular fibrilla. When the ventricles are fibrillating, they are not contracting and generating blood flow and blood pressure. This means no oxygen gets to the brain and other vital organs. Within 2 minutes or so the brain will suffer irreversible damage or death if circulation is not restored.