I don't know if anyone has read the "Wheel ot Time" fantasy series by Robert Jordan. Epic doesn't even begin to describe it. 14 books in total - ranging in the neighborhood of 800-1200 pages in hardcover.
Jordan was a funny looking little troll of a fat man like Martin. My wife and friends and I who loved to discuss the books would often joke that Jordan was going to die before the books were finished. It was obvious around book 7ish or so that the story was so massive; the main characters so numerous; the created world so sprawling and complex; the numerous plots so simultaneously interwoven and disparate; that the next book was never going to be the last book.
Well, he died. Fortunately, before he did, his publishers demanded that he provide a coherent and specific outline of how the story was to play out and ultimately end. He did it, and after his death, the publisher (his wife) auditioned and hired a fella named Brandon Sanderson to faithfully complete the last three books in the series. He did a really, really good job. You really couldn't tell it wasn't Jordan's work, with few exceptions. They even credit the books to Robert Jordan, with Sanderson as a secondary credit.
I think the same thing is going to happen with George R.R. Martin. I don't want to predict someone's death - and I guess I'm not even doing that. I'm just saying that he's going to die before the books are ever written. He'd be smart now to be creating detailed notes and making sure his estate has legal rights in proper order for any derivative work created after his death.