We haven't perceived non-existence with the senses or retained memory of it in our brains, we never experienced it in the way we experience life.
People fear death for several reasons besides fear of the unknown, these include fear of causing distress to the people they leave behind, fear of a painful death and fear of not accomplishing their goals in life.
I don't really understand how your conclusions relate to the hypothesis but to be frank this idea feels half baked to begin with with such large stretches being made.