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From a physical standpoint, our base energy/matter cannot be destroyed anyway. Even antimatter annihilation is just superefficient conversion between types.

So in that regard, there isn't much to worry about. You'll always physically exist for literally eternity.

The issue is that our egos are, to the best of out knowledge, a product of information processing, and information can be created and destroyed.

So there's a sort of duality going on. We'll exist forever in some form or another, but the part of us responsible for our self awareness will not. That's where the worry comes from.

That's why people like Kurzweil and Musk put so much emphasis on computing power. Their end goal is to allow information to be essentially indestructable as well, and use that newfound invulnerability to safeguard our "information" selves, i.e our egos, around for eternity as well. Or at least until the heat death of the universe or we discover a way to cheat it.