It's immense, and clearly a big category in every major book outlet, which is terrific. Okay, half of it might be taken up by Star Trek and Star Wars spin-offs, but there's clearly space in sf for the real literature. By its nature science fiction is never going to be as formulaic as other genres, like mysteries, and it's never going to be so bound by settings the way westerns are. Therefore it just can't lose its appeal, and it's a genre that can adapt with the times. You could say it is the genre of our times.
Science fiction has a great future.