The point is that everyone is doomed (even if you imagine we can survive the civilization-murdering tools we've cobbled up, we can't outrun physics), but that even at our most vulnerable, since the book occurs during a period directly after Armageddon, it is possible for some goodness in us to persist.

I don't want to spoil, but the optimism isn't for the characters, it's for we the reader, and the species.

The thimble of fire joins the wider flame. Goodness survives even there, and even then.