Here's my speculation: American SF is going through a gloom-laden period induced by external social conditions, much as British SF did in the 1947-79 period (and differently, in the 1980-92 period). Extrapolative SF is often used by writers as a mirror for reflecting our concerns about the present on the silver screen of the future. "Invasion of the Body Snatchers" and "The Puppet Masters" were artefacts of the late 1940's/early 1950's paranoia about communist infiltration. "Fugue for a Darkening Island" was a dismal if-this-goes-on dirge played to the tune of Enoch Powell. "Neuromancer" was 1980's corporate deracination hooked up to an overdose of MTV, mainlining on hidden assumptions of monetarism. When SF is at its most overtly predictive -- especially when it speaks of the impending future -- it is talking about the present, capturing the zeitgeist and projecting it forward.
n American SF today there is a huge surge in the proportion of alternate history counterfactuals: as James Nicoll notes, AH is often used as a consolatory literature that, at its worst, says "we go back in time and make history happen the way it should have happened! Yay, Us! In a completely contrived scenario, we can win!" The boom in fantasy probably needs no further explanation. Ditto the military-SF field, which at its worst reflects the self-indulgent imperialist excesses of the British penny dreadfuls of the early 20th century.
what's almost totally absent is convincing near-future SF about a future America that is anything other than a dystopic rubbish dump. Bleakness is the new optimism. Writers living in the USA today just don't seem enthusiastic about the near future in the way that they did as recently as the 1980's, where at least the cyberpunk future of cliche was a vaguely habitable pastiche of the globalized present. They are, in fact, exhibiting the same canary-in-a-sociological-coalmine malaise as British SF writers of the 1960's and 1970's.
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