And by that I don't mean that science fiction is stories set in the future.
At this moment in time, in purely commercial terms, taking the genre as a whole, fantasy is outselling science fiction. Mark Charan Newton gives some reasons why on his blog here.
But that means what, exactly? That sf is at risk? that it's dying? that if this terrible state of affairs keeps up, there'll be no more science fiction?
Of course not.
These days, I suspect it's wrong to even call sf a genre. It's more of a culture set. Its styles and tropes, anything which might readily identify it, have been picked up by other genres, have been spun out to create yet other genres, have become in many respects a significant part of our cultural landscape.
(This doesn't mean I buy into the "we live in a science fiction world, so people don't want to read it" argument. The 1950s - atomic bombs! - and 1960s - the Apollo programme! - were pretty much science fiction worlds, and the genre was going strong then.)
As I said, science fiction has spread out into a number of diverse cultures - some it has infected, some it has generated fully-formed from its own brow. Cyberpunk, steampunk, military sf, for example. It has invaded popular film and television and computer games.
So sf is no longer a monolithic genre or culture. Add up everything that can be called "science fiction" and I think you'll find it outsells fantasy. It's not just literature anymore. Neither, of course, is fantasy - just look at the success of The Lord of the Rings and Harry Potter films. But fantasy is not yet as pervasive as sf in the western cultural landscape.
(Yes, some forms of fantasy play a significant role in western culture; but not the form usually identified as fantasy literature - unlike that which is usually identified as science fiction literature.)
So yes, science fiction is the literature of the future because it is not just literature. It is a culture, it is pervasive. It is populating, and may soon dominate, our cultural landscape. Science fiction is not just the literature of the future, it is the future.