GRRMs very early planning for the series was to write it as a trilogy. But once he got into writing the war of the 5 kings, he realized he had much more story to tell.
Hes not really into doing a lot of neatly structured planning and outlining for his writing. Hes compared his writing to gardening a lot in the past: he likes to plant the seed of a story, water it, and see how it grows when he puts time into it.
Thats a big part of the reason hes taken so long since the 3rd book to publish. He originally intended to just have 5 year gap after Storm of Swords and go straight into Winds of Winter. But he found himself having to spend a lot of his writing explaining why the characters were where they and how they got there, so he ended up saying screw it and writing a book about the 5 year gap. But then that really kind of snowballed on him and he had too many pegs for one book, but no place that made a lot of sense around the middle as a stopping point, so he and his publishers decided to split the book up geographically and have books 4 and 5 taking place more or less concurrently.
With the 5th book, he originally intended to end with the battles in Mereen and in Winterfell, but again the book was getting too long and publishers talked him into taking what hed written for those and pushing it to the next book. When the 5th book was published, he already had a head start of 100 or so pages on book 6 because of that.
As for someone else finishing his work, Im kind of skeptical. GRRM May come around, but hes been somewhat hostile to the idea of someone finishing his work for him before. Also, because of his writing style, Im not sure whatever wed get would be that reflective of what GRRM would end up writing anyway. Hes had some broad strokes planned for a while, but hes known for making so many changes as he writes his characters and gets into their heads. Im not sure that could be replicated by someone else very accurately (though I wouldnt mind too much of it meant we get closure on the story).
