That's the point? Anything that has survived for a billion years in a form that could still be considered roughly the same organism or a close descendant must

That's the point? Anything that has survived for a billion years in a form that could still be considered roughly the same organism or a close descendant must have a relatively stable biochemistry and gene pool that is capable of surviving an extreme range of environmental changes - as close as you can get to a holy grail in natural selection. Obviously its survival would not be inevitable; not even stars can survive their own age.