He's been quoted saying that he could have seen several more seasons being done (which would have kept with the pace of the other seasons that are directly based on the books), and instead they truncated it to two even-shorter-than-usual seasons.
What I don't understand is if the show writers wanted to move on to other projects (which they did), why didn't they just hand off the reins to literally any of the hundreds or thousands of fan theorists on the internet who could have written a much better ending with George's notes? I mean, some of the theories I heard were 1000x better than what we got.