I’ve seen this post go around and the idea that “adult fantasy has to be dark and gritty” nowadays is… weirdly surprising to me. A Song of Ice and Fire (the book series) was dark and gritty as part of its deconstruction of the fantasy genre; it takes a lot of wish-fulfillment-y tropes that were common in fantasy up to that point and tears them apart mercilessly: the power of honor and nobility, the magical chosen hero who comes in and saves everyone from chaos despite their utter inexperience, the loyal common people who are content with their lot and love their wise king. When the books were published, their grimdark “realism” was actually pretty refreshing as a different look at a medieval fantasy world from the rosy, unexaminedly pro-monarchy one common in fantasy. (Disclaimer that I don’t really know if this all is what GRRM actually intended with them, but it’s how I interpreted them in the early 2000s.) And the series was popular among fantasy readers from the start, but I don’t think it really made the genre become dark and gritty along with it yet.
And then for whatever reason, the TV adaptation just hit exactly the right chord in this decade’s zeitgeist, and I guess the vast majority of viewers, who probably hadn’t had much prior exposure to non-Harry Potter fantasy, took it as What Fantasy Is? The deconstruction of unexamined fantasy cliches from three decades ago has become the source of the new unexamined fantasy cliches today. But that’s pretty much how artistic change works.