So these examples are actually difficult to extrapolate from for the OP’s question as these happen to be examples of some of the specific brain changes that people don’t notice.

Was just chatting with a neurologist on this.

The brain sort of accommodates for the area based vision loss. They’d mentioned an example where someone who had effectively lost half their visual field in this way would draw a clock that was round on one side but went straight up the middle instead of extending to the other side, and it looked ‘right’ to them when double checking it.

As well, the one area of language that can get messed up without them noticing is the one responsible for understanding word meanings, as it kind of ends up as a deficit in both what they say and how they hear it. They’ll just chatter on with what everyone else can recognize as nonsense words but they have no idea.

Whereas the other area that’s related to the ability to form words when speaking they very much notice. They can only speak really slowly, but they know they are speaking very slowly and get very frustrated.

So the deficits your brother had were luckily the ones that didn’t end up being very noticable and frustrating for him, but that was possibly more a mechanism of those specific deficits.

OP’s question is a fairly different process, but I’d be inclined to agree that in the severe cases there’s not much concept of death. But in part, that’s because in severe dementia cases there may not be much of a consistent concept of self to consider as ending in the first place. It’s quite degenerative by the very end, and the body still being active and reactive outwardly doesn’t necessarily mean a subjective and continuous consciousness with an accumulated sense of self is still inside the way we experience it when healthy.

Degenerative brain disorders that attack cognition are scary as shit and I kind of wish more people realized this better as we might have much better laws regarding dignity and frequency in organizing end of life if it were so.