[stonetools]

Writing is hard work; writers are often compelled to follow-up whenever a good story or compelling characters present themselves so sequels and series aren't always about milking a popular character or milieu for a few extra bucks; sometimes it's about a muse that just won't be denied.

If you look at this ARTICLE I have a lot of sympathy for the idea that the story some times takes over once the author begins to write .

After three chapters, he had a vivid idea of a boy seeing a man's beheading and finding direwolves in the snow, which would eventually become the first non-prologue chapter of A Game of Thrones.[21] Putting Avalon aside, Martin finished this chapter in a few days and grew certain that it was part of a longer story

Its incredible that the the entire ASOIAF world grew out of a single scene. I think we have to reconsider DDs insistence that GRRM should just get to the point already and should have finished the thing. I believe GRRM did originally plan to get the job done in three books, but the story just took on a life of its own , especially this:

After A Game of Thrones, A Clash of Kings, and A Storm of Swords, Martin originally intended to write three more books.[18] The fourth book, tentatively titled A Dance with Dragons, was to focus on Daenerys Targaryen's return to Westeros and the conflicts that creates.[24] Martin wanted to set this story five years after A Storm of Swords so that the younger characters could grow older and the dragons grow larger.[32] Agreeing with his publishers early on that the new book should be shorter than A Storm of Swords, Martin set out to write the novel closer in length to A Clash of Kings.[26] A long prologue was to establish what had happened in the meantime, initially just as one chapter of Aeron Damphair on the Iron Islands at the Kingsmoot.

That initial plan did not work and he had to re-conceive the arc of the story at midpoint. You can't predict stuff like that, you just have to do it.
I also have to re-consider my rather cynical point about recycling. The ASOIAF world is so rich that GRRM could spend the rest of his life just exploring the story possibilities . I could easily imagine a standalone book or even series based on the Free Cities mileiu or on pre-Doom Valyria . Those would count practically as "new worlds" since they are so different from Westeros in the era of ASOIAF. I would favor GRRM revisiting the ASOIAF world after the present story arc is done to explore new stories.
(He is already doing this with the Dunk and Egg stories but this is really more of the same, rather than a significantly different setting like a Valyria setting would be. )