<snipped for emphasis>
Death used to mean when the heart stopped beating because it was what invariably happened before we had the capacity to plug people into machines and keep things going after parts of them died. Before the advent of the modern brain scanning and modern emergency medicine nobody was ever even diagnosed brain dead because there were none. Once the brain died, the heart died too. The only way to ascertain that anyone had experienced an irreversible cessation of brain function and had no brain perfusion was to see that yup, he's not breathing, his heart stopped, he's dead.
People are free to believe in prayers and miracles but an expectation of a miracle is not a medical indication to continue treatment of a deceased person. It is a religious thing, so perhaps there are churches who'd like to step up and provide this for the people believing that a miracle will happen, but it is not a job for the hospitals.
This quote from a Catholic theology site sums it up imo.
http://catholicmoraltheology.com/what-are-we-learning-from-the-jahi-mcmath-case/
"Of course, God can work a miracle. But prudential medical decisions cannot be made based merely on the hope that God might choose to act miraculously."
God chooses to resurrect brain dead people very infrequently and there is no brain scan or a lab test that would let hospitals know that He is going to raise Jahi McMath from the death with any larger probability than any other brain dead person.
JMO.