Then the author uses this knowledge to write a story about how the world will be in 2050, 2100, or 2500. Sci-fi that is merely fantasy with a veneer of spaceships and robots isn't what I'm looking for.
I didn't say anything about it being optimistic or pessimistic, just that most writers seem to inject their personal politics into their writing, which is a real distraction from the worldbuilding. Children of Men is particularly pessimistic, and I think anyone reading it would agree.