People are often not particularly clear on exactly what they expect will survive death and how.
Chemicals or physical interaction with the brain can change its state of awareness from fully aware to not aware at all, and various states of partial awareness in between.
Damage to the brain, like in the case of head trauma or a stroke, can alter a person's personality and/or erase their memories. Personality is also related to the development of neurons, so if a person has a genetic or congenital problem, or they are abused and denied stimuli, they can fail to develop a complex enough set of neural connections to have an intelligent personality.
So various things are linked with the brain in a correlative and oftentimes causal manner. If that brain is destroyed, then what do we expect to happen?
The more vague an afterlife description is, the less there is to stay against it. Consciousness itself hasn't been figured out scientifically yet, other than that it can be diminished, shut off, or altered in a living brain by the aforementioned chemical or physical means.
If consciousness itself can survive death even though physical or chemical methods cans shut it off, but does not bring with it personality, then that's very different from what a lot of religions teach. Similarly, if consciousness itself can survive death but there is no biological cognitive machinery to be able to form memories from moment to moment and link quantum instances of awareness into a coherent stream of awareness, then it's essentially as though awareness doesn't exist, because it exists in bursts for an infinitesimally small duration without links between.