I've been tracking the results of digging into the trends influencing the publishing of science fiction and fantasy since the original impulse to dig into Appendix N of the 1st edition of the Dungeon Master's Guide for Advanced Dungeons and Dragons in the hope of explaining why so much changed in tabletop RPGs since 1974.
This lead to me reading the Castalia House blog's series on Appendix N, itself in part inspired by Grognardia's digging into that list of old genre fiction that informed Gary Gygax, Dave Arneson, and other early tabletop RPG creators in the late '60s through the '70s, and recently came a post that made some interesting conclusions. Vox Day already quoted the most relevant part of the post, so I'll direct you to his post on this matter if you want his remarks (which mirror my own thoughts). The same applies to his remarks regarding Brad Torgersen on the reality of SocJus tolerance..
What it comes down to is that 1980 seems to be a breakpoint.
Look, I can go on about this, but that would unnecessarily duplicate effort already spent in the linked posts and their comment threads. Yes, go read the comments, because you're getting good commentary and additional information that the posts proper omitted. What matters is this: The folks who left in 1980, and their descendants, are coming home and making SF/F great again.