First of all, I don't "hypothesize" anything - I am just pointing out what seems to be the obvious answer to "what happens when you die?". When the brain is damaged or undergoes chemical changes, we know that consciousness seems to disappear. There is no strong evidence to think this is different upon the death of any organism.
Second, I find your puzzlement with the fear of nonexistence utterly incoherent. Humans are born with the desire to live. We don't want to stop existing. Stating the obvious truth that one can't experience things when they are dead is completely void of content - the point is that we fear our eternal end WHILE we are alive because we'd like to continue on and be healthy again.
>When the brain is damaged or undergoes chemical changes, we know that consciousness seems to disappear. There is no strong evidence to think this is different upon the death of any organism.
People often have dreams under such circumstances, and there are many who have died and been resusitated that report having wild experiences inbetween.
>we fear our eternal end WHILE we are alive
Again, you're mythologizing/hypothesizing here with your use of the word "eternal".
But yes, when confronted with immediate life threatening situations, all animals including humans have a natural fight or flight instinct. However, not all humans (and seemingly no animals) have such existential dread like you do of a future of eternal nonexistence. Your fear is a result of your materialist reductionist mythology.
The simple fact of this discussion is: no one knows what death is truly like, so we can only extrapolate, hypothesize, and mythologize.