Nor do they show the same thing with defintiive proof.
In fact, autopsy is the only way to find out what exactly was going on in the brain. So, for example, many US insurers used to exploit this loophole to get out of paying for Alzheimer's patients: they required a definitive diagnosis before they would pay for care. Only way to diagnose Alzheimer's definitively is an autopsy. MRI or brain scan don't work.
So yeah, autopsy's a good idea; after all the medical examinations and tests by people without a theological axe to grind show that she is, indeed, vegetative, an autopsy will put to rest the line of arguments that she might improve. Who knows? It might even reveal that a conscious human bieng was murdered.