All of which is true, and has certainly been true for all kinds of crossover and slipstream works since 1973, if not much earlier. But you can find a different kind of secret history of sf in another book, Sin-a-rama: Sleaze Sex Paperbacks of the Sixties, which collects together all kinds of lurid covers and essays by publishers and authors, including one by Robert Silverberg in which he describes how he wrote 150 softcore sleaze novels in five years for fun and profit. Harlan Ellison and Marion Zimmer Bradley wrote sleaze novels, too; so did mystery writer Donald Westlake, and a number of other well-known authors. At the time, Silverberg explains, ‘A dozen or so magazines for which I had been writing regularly ceased publication overnight; and as for the tiny market for s-f novels . . . it suddenly became so tight that unless you were one of the first-magnitude stars like Robert Heinlein or Isaac Asimov you were out of luck.’ Silverberg turned to the sleaze trade as a way of earning a living, and discovered that it was also a valuable apprenticeship: ‘It isn’t just that I earned enough by writing them to pay for that big house and my trips to Europe. I developed and honed important professional skills, too, while I was pounding out all those books.’
Sf publishing has always been a chancy, hand-to-mouth affair for most. It imploded again in the early 1980s, and there are signs that it’s about to implode again. And because they can’t hope for sinecure positions in creative writing in universities (although that’s changing, now), sf writers have always been ready to turn their hands and minds to the kind of writing that can be churned out quickly and profitably. In the golden age of the pulps, the 1940s and 1950s, sf authors like James Blish or Frederik Pohl were capable of banging out one story for Amazing in the morning and another for Stirring Sports Stories in the afternoon (and barely made a living at it - see for instance Pohl’s fine memoir The Way the Future Was, or the roman-a-clef opening of Blish’s Jack of Eagles, in which the penniless hero pours tea on his cornflakes because he can’t afford milk). While Silverberg et al were working in the titillation trade in the US, over here in the UK Michael Moorcock was editing New Worlds with one hand and writing Sexton Blake adventures with the other, while many of his contemporaries were writing westerns, biker novels and, yes, sexploitation novels. A little later, Kim Newman and Neil Gaiman worked for the British soft porn magazine Knave. And sf writers today are also working in comics and graphic novels, novels based on role-playing games (Kim Newman and a slew of authors associated with Interzone in the 1990s wrote innovative and highly successful short stories novels for Games Workshop), film tie-ins . . .
These days, of course, there are plenty of sf writers who didn’t come up through pulps, or via sf fandom. But it was in the febrile arena of pulp sf that many tropes and imagery in common sf toolkit was generated and shared and elaborated upon (apart from all those ideas invented by HG Wells and Jules Verne). And while sf can sometimes aspire to the condition of literature, just as literature can sometimes aspire to the condition of sf, and while there are plenty of so-called literary qualities which all writers should aspire to master, and every kind of bad writing in whatever field should be rightly despised, there are values outside of the literary canon that have their own intrinsic worth.
The themes and tropes of sf have become part of pop culture and the happening world. Most of the writers in the sf genre use them as if they were real, most writers outside it use them metaphorically or allegorically. Both can produce works of lasting value, but one is looking forward, and the other is looking back. Think of these two secret histories as poles of a magnet, with sf inhabiting the field lines stretched between them: a continuum in which the only borderlines are those writers choose to draw around themselves.