The end of human life doesn't necessarily need to be the main focus. It's okay if the book only spends 1-2 pages talking about it out of several.
One potential angle is:
In 800 million years, photosynthesis will no longer be possible on Earth and all plants and animals will go extinct. About 200 million years after this, the intensity of the sun will have risen to the point where oceans will have boiled away, making above ground complex life impossible.
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