Glen Cook was killing off major characters back in the 1980s.

JG Ballard, John Brunner (and others) were writing miserabilist dystopias back in the 70s.

Also, Dune, which is certainly not a "The good guys win" story.

And movies like Soylent Green.

The Disney trope turns out not to be a trope at all. The good guys win/hero's journey is much older than Disney, and it's popular because we'd like to be believe it's how the world works.

But GoT didn't do anything really new, except wrap some porn around the various murder plots.