John Scalzi is absolutely right in his recent film post that there's little chance SF will go the way of the Western. The genre of the Western is hamstrung by being a celebration of a slight historical period, perhaps some two decades at the end of the 19th Century, before barbed wire segmented the plains. It's also replete with distasteful racial and ethnocentric biases.
SF does face another issue however, one of obsolescence. I don't mean that it'll become obsolete as a genre, but that individual works will. Each SF novel or movie is an extrapolation from the technology of its time. As science and technology progresses, the future diverges more and more markedly from earlier predictions.
Futurism is an impossible art, as sites like Retrofuture Today highlight swimmingly.
When we go back to read SF from the past, we note the anachronisms. Today we're surrounded by the internet's new media, yet personal flying machines never took off. 1984's 1984 is twenty six years past now, and yet the world hasn't yet descended into a fascist surveillance state. So they ring a touch dull, the inaccuracies force us out of the narrative.
That is, unless we view them all as fantasies, with a little 'f'. The great virtue of High Fantasies like Lord of the Rings is that as examples of a past-gazing genre they can't ever become obsolete: they make no predictions.
(As an aside, I've often wondered if that's part of the reason why Lucas wanted Star Wars to be explicitly placed "a long time ago in a galaxy far, far away". No chance of obsolescence or broken predictions there!)
If we discard the apparent predictive value of SF, most strongly held in its "hard" version, we can perhaps recover the narrative force of the story when the future has left its predictions in the dust. Does it really matter that Bradbury's Mars of the Chronicles bears no resemblance to the actual planet? Perhaps it does, for some, but not so much if it's seen as fantasy.
In a similar vein, I wonder if that's some of the pull that readers and writers feel from subgenres like Steampunk. Since they're already backwards-gazing, they short-circuit the problems of anachronism and obsolescence by embracing them as part of the narrative: of course there were no towering steam fortresses in Victorian Europe; it's all fantasy.
I'm not sure how long lived such a move can be, though. SF, by its very nature, is relentlessly forward thinking. But it is an intriguing way to approach the problem.