Interesting. So you are saying that if the soul were immaterial, then it should be able to directly interact with the physical world despite the body being incapacitated due to the brain's failure. However, most of the widely accepted philosophical beliefs that claim that there is a transient soul hold that it does not interact with the world directly, but through the body.

Dualism, advocated for by Plato, says that the soul is the center of thought, self-knowledge, and being; the body is merely a shell through which the soul interacts with the material. If this is true, then no matter what happens to the brain, the soul remains intact. However, the vehicle the it interacts with the world is crippled, making communication between the soul and physical world more and more difficult. All memories are either attached to the transcendent soul or, if forgotten, accidental qualities that do not make the soul what it is.

Aristotle advocates for Hermaphroditism, which states that the soul is the "blueprint" of the body. For example, since your soul contains the essence of humanity, your body contains humanity in its form and function. Because you contain humanity, you are capable of doing all things intrinsic to "humanity" such as acquire memories, think, and walk. However, the intrinsic ability to do these things does not mean that one HAS to be able to do them. Though our humanity makes man capable of playing an musical instrument, it does not mean the he has to in order to be human. Non-musicians are just as human as musicians. In this worldview though the humanity found in man's soul makes him capable of proper mental function, he does not have to have perfect mental integrity in order to maintain both his soul and humanity.