It was like new authors who didn't write in the grimdark style basically stopped being published for several years, though.
I remember it pretty distinctly. And the authors which were grimdark were promoted and marketed vastly more aggressively than older authors who weren't, for that period. It definitely had a significant and long-term impact on the fantasy landscape. There are authors successful today, who had the whole "grimdark" thing not happened and been pushed by publishers, might never have been successful. There are others who it impacted the career of.
And saying that "the old stuff doesn't go away" is totally wrong with fantasy particularly. Some stuff which was absolutely huge, earth-shatteringly influential, in the 1970s and earlier 1980s was basically close to forgotten by the 1990s, and is nearly completely forgotten now. Case in point, Michael Moorcock. He was a goddamn titan up into the early '80s, even non-fantasy critics and stuff were talking about him. Today? Most fantasy readers have never even heard of him, let alone read one of his books. He's a large part of the reason D&D and Warhammer are the way they are, but you'll hear 30-somethings who've never heard of him blithely asserting both were influenced more or less solely by Tolkien (which with Warhammer particularly is just completely insane nonsense of the most ignorant kind - but then other 20-something and 30-something people slap each other on the back and all agree about about). It's a travesty but it's a thing that's already happened.
Some fantasy stuff survives better (it's hard to predict which, it's certainly not related to how influential it is), but an awful lot of it absolutely does "go away". Hell, hardly anyone under about 35 seems to have actually read any pulp fantasy at all apart from maybe a few Conan short stories if you're very lucky.
Moorcock is an author, not a theme or sub-genre. That's what I was talking about. Oh yeah, and Moorcock is still publishing - he has a following. Maybe newer fans haven't heard of him, but he's known by everyone with anything more than a surface knowledge of fantasy.
Moorcock was instrumental in establishing sword & sorcery, or reviving it in the 60s. And it hasn't gone away. It may not be as prominent as other sub-genres of fantasy, but it still has a strong following, and has also influenced grimdark.
That said, I hear you about him being under-appreciated in terms of his influence. In my mind, and I think in the minds with scholars of the genre, he's a giant.
And I hear you about stuff fading, and younger folks not having a historical context. That's probably just due to casual fandom: most people read The Latest Thing, and only serious fans look back beyond stuff published more than 20 years ago or so.
But my point is, even if things rise and fall, a lot of stuff comes back around. That's how the fashion world works (as far as I understand it).
We also live somewhat in a "post-genre era," where there is less room for new territory to be discovered, and a wealth of old stuff to sort through and re-vitalize in new ways. I mean, have you ever noticed how the cultural themes of the 20th century are more vivid than in the 21st century? Maybe it is "recency blindness," but I just don't see the 2000s or 2010s as having as vivid a "cultural signature" as the 1920s - 1990s. It is almost like we, or at least Western culture, tried everything out in the 20th century, and the 21st century is more about re-combining and integrating, with less new ideas coming in. Very postmodern of us! Just a hypothesis, though.
p.s. Seeing as you're obviously a sword & sorcery fan, have you checked out the recent survey of the field,
Flame and Crimson? A fun book.
