I'm surprised no-one's mentioned it already, but clearly the decline in SF all the fault of Lightspeed Magazine and women.
Women Destroy Science Fiction.
And SF is going to continue to decline, because in 2015,
Queers Destroy Science Fiction.
Alas, poor Science Fiction.
(For those who don't follow links, the links are to two kickstarter campaigns by Lightspeed magazine for special issues edited and written exclusively by women (2014) and 'queers' (2015). The titles are, of course, tongue-in-cheek. The 2014 issue was very good, and they offer great perks for funding the 2015 kickstarter.)