A spirit without a brain is non-functional. The human spirit provides intellect to a human brain which is part of a body.
Because the human spirit is non-material it is not subject to injury or disease, but if the brain is not functioning God retrieves the human spirit so it can be mated with a different brain when need be.
Spirit and brain are not the same. You are trying to dissect the human body by sending some of it to heaven and some to the grave.