My definition of SF is based loosely on Kingsley Amis', in his book "New Maps of Hell", a literary criticism of early SF. No doubt there are other definitions, and they are all equally valid--after all, definitions are, by definition, both subjective and arbitrary.

I feel SF is that branch of literature that concerns itself with how technological and other changes will affect us, both psychologically and socially. In other words, SF examines our society by extrapolating contemporary trends into the future, or by observing how our society will change when faced with a technological or cultural shock. By this definition, Verne's "20,000 Leagues Under the Sea" is not really SF since it is primarily a psychological study of Captain Nemo wrapped around a pure adventure story. Verne made no effort to try to imagine how submarine warfare would affect international conflict forever, on a strategic scale, and his guesses on how submersibles would be employed tactically were way off the mark.

OTOH, Wells' "War of the Worlds" is SF by this definition because it examines certain Victorian attitudes towards Imperialism and European cultural supremacy by turning the tables and making Britain the "natives" under attack and occupation by a superior foreign technological civilization. This definition is not a judgement of literary merit, both Wells and Vern wrote terrific books. The first two "Alien" films were ground-breaking entertainments, true works of art in my opinion, but none were really SF by my and Amis' definition.

A film like "Avatar", though, is definitely SF, since it critically examines contemporary themes in a futuristic setting: cultural and technological imperialism, and commercial impact on an aboriginal culture. However, dramatically, it was a lousy movie, a bit too preachy and self-righteous, even for those who might accept its point of view. In contrast, Herbert's "Dune" is an amazing work of prophecy and analysis, dealing with themes like messianic religion, industrial and political exploitation of tribal societies unfortunate enough to be blessed with valuable resources, and how they can be dragged into superpower confrontations. "Dune" is history, not through a microscope, but through the telescope.

I also believe we still need SF and for the same reasons you do, to look into the future. The novel and the film can be used to study the human condition, both individually and collectively, by speculating how we react to change, particularly (in the case of SF) technological change. Learning what happens to people under stress is why we have literature in the first place.

It isn't usually possible to predict new technology, new science, or how we will respond to it. But SF reminds us that there will always be unanticipated and unforeseen consequences. We tend to forget that.