*I am speaking here, as I usually am, of the entity that named itself that: i.e., the field of literary endeavor (and, later, other media) that began in, then branched out from, the American magazines — which is not to say only American writers and only ever American magazines — including the novels and other books that eventually began to be published, and including also those semi-autonomous areas that defined themselves (explicitly or otherwise) in part or entirely as reactions against this field.
1910s-20s: Science fiction establishes itself as a distinct, self-aware entity. It finds itself in the pulps not by choice but by default.
1930s: Sf refines this self-image, establishes its own protocols separate from those of the rest of literature (including both the "mainstream", broadly defined, and other "genre" or "category" fiction), and accomodates itself (usually enthusiastically) to its pulp identity.
1940s: Further refinement, now in the form of channeling previously disparate strands into one specific path (i.e., Campbellian sf dominates both in and to a lesser extent out of Astounding, and either way Astounding dominates sf).
1950s: Explosion as that one path becomes increasingly untenable/exhausted.
1960s: Post-explosion burnout. Flailing, sometimes successful, attempts at rapprochement with the interests of the "mainstream" (including the "avant-garde").
1980s: Backlash leading to an ultimately successful counterrevolution.
1990s: Entrenchment of counterrevolutionary gains. Revolutionary impulses allowed expression as long as they are placed and contained within the larger counterrevolution.
And then 2000s-10s? What might be said of the current and previous decade? I'm tempted to say that they are essentially "the long 1990s." But they do have their own distinctive character, one defined perhaps... by contradictory urges toward amnesia and nostalgia? By a full and almost relieved embrace, at long last, of the devotion to formula implied by the word "genre"? By a vehement denial that there should or could be anything distinctive about sf beyond a suite of decontextualized and impoverished images, a denial that there exists any value apart from the "literary"? By a process some might call "evaporation" but which I would say is more like metastasis, by which sf has become inseparable from the mainstream?
The age of conquerors (not a term of praise) and inconsequential scribblers (not necessarily a term of disparagement), with nothing in between? Of fracturing over both arcane non-differences and genuine life-and-death matters, with little distinction made between the two? Of internationalism, in both its cooperative and imperialist forms? Of diversification, both for good ("inclusivity") and for ill (assimilation, "diversification" in the sense of a stock portfolio or a multinational corporation)?