This is why the "D&D had GRRM's notes to work from" argument doesn't hold much water with me. Because you know who else has GRRM's notes for continuing the story? GRRM! And yet 10+ years later, the existence of those notes hasn't made his work any easier or faster to produce. Obviously just having GRRM's general direction for the story can only do so much in terms of getting actual material down onto a page, and then in front of a camera, as GRRM himself can attest to by now.
The time between the release of Dance with Dragons and the present is longer than the entirety of Game of Thrones run time!
It's very likely several seasons of the prequel series will be released before the Winds of Winter is even finished (if it ever is)!