Yes if the heart has flatlined completely then it won’t be started again by AED. But if they are in cardiac arrest then the heart rate is erratic and doesn’t function normally but can be returned to normal with a defibrillator.
If you’re doing chest compressions to save someone who has stopped breathing there roughly a 10% chance they will survive but a 0% chance if you don’t So it’s often worth it anyway.
However it does keep the blood flowing and can prevent brain damage until a defibrillator is available which has a much higher chance of saving someone who has stopped breathing.
The heart is also protected by the lungs and breast plate.
A lot of people think that regular human transplants are problem free but the vast majority of them are rejected by the receiver. If it’s for a life saving procedure it can only extend the life span by so much…