Stranger Things will end with the original kids alive and the threat to Hawkins ended, once and for all, for instance.
Is this a prediction or based on an outright statement from the showrunners?
Because based on the previous shows, the general tone and approach to drama, and the movies/shows it is emulating I would fully expect 1-2 of the kids to end up heroically dead (or vanished into the Upside Down with no seeming hope for rescue) in the final episode. Obviously if it's a statement from the showrunners that overrides that and is surprising.
And dark fantasy has been rather prominent since about 2008. Zombie apocalypses, the latter Harry Potter movies, Game of Thrones, The First Law Trilogy, Stranger Things, Berserk and Attack on Titan, etc. Even the more recent Marvel movies
It's more prominent as a subgenre than it was in the '80s and before, but there are a couple of things worth noting:
1) The
default for fantasy was a lot darker back then. Fantasy,
on average, has got lighter since the 1980s and earlier. Especially if you're looking at novels, TV and movies. Bridge to Terabithia isn't "dark fantasy", for example, no matter how upsetting it is.
2) All the stuff you list is the exception, not the rule, and you're listing stuff that's wildly successful next to stuff that's a moderate niche success, and stuff that's slightly dark - i.e. has some bad things happen - but isn't really darker than "normal" fantasy in the '80s, like Harry Potter - next to stuff that's nearly pitch black, like Berserk or the more downer zombie stuff.
And "the more recent Marvel movies"? No. Absolutely not. Of those only Multiverse of Madness could even arguably be considered dark, and I'd say that was balanced by the extremely light-side No Way Home (i.e. where MCU Peter Parker finally finds his real compassion and moral center, and where all the bad guys get sent home healed*, not murdered). Wakanda isn't dark, given the ending(s). Thor 4 wasn't dark - a character dying doesn't make a film "dark fantasy", for god's sake. Quantumania is, I presume, not dark - certainly no-one I know who has seen it has suggested it was - the main comparator has been Star Wars.
Well, there's always been some dark fantasy. It just feels like it's really dominant right now. GoT was published in the 90s, but it didn't reach the height of it's popularity until the mid 2000s. The First Law trilogy took off then, too. Zombie apocalypses, Nolan's Batman franchise, The Hunger Games, whatever Zack "Batman kills" Snyder is filming, etc. The Last of Us was just made into a series so we're back in that rather miserable universe. Like something has to be going wrong when The Witcher almost feels like it's refreshingly optimistic.
This is all on you, frankly.
You're completely failing to differentiate stories which are incredibly dark and horrifying from stories which are ultimately hopeful, even joyful, but involve some degree of suffering on the way. They're not the same thing. The First Law is pitch black. Everything is bad. Everyone is bad (to varying degrees) and importantly, everything is going to get worse. The Hunger Games, on the other hand is extremely hopeful. Love changes the world for the better. Love and fellow-feeling/solidarity sparks a revolution. A dystopia is overthrown through bravery, sacrifice, grit and decency. The main character lives happily ever after with her boyfriend in the big house she dreamed of.
If that's not a happy enough ending to count as "fairy tale", then that's on you, but it's not a "dark" ending by any means, and they stories are hopeful about humanity's solidarity, decency and ability to overthrow tyranny. Even the hearts of the corrupt and venal, or totally superficial-seeming are touched and changed by love and hope for a better world in Hunger Games, for goodness sake.
Mentioning Hunger Games in the same breath as The First Law or GoT or even The Walking Dead is absolutely nonsense and illustrates a serious failure to engage with the themes and ideas of the works, and instead a focus on entirely superficial elements.
And even then - these are wildly in the minority, fiction-wise, here in 2023. You might have argued it more persuasively like, a decade ago.
* = I get that if we're intentionally literalist eejits they're potentially just gonna die, but that is clearly not the
intention of the writers, given the emotional context and so on. The film was already too long, guys!