GRRM has a stranglehold on his publishing contract. He's not concerned about money, so much as legacy. He didn't leave the show on good terms after season four (three?) and from that point didn't really care if they couldn't keep up with the quality of his writing.
Anyway, if the books do come out before everyone in the series is too old to play the parts, they could go onto a Season Nine, with GRRM's involvement and put a pretty satisfying bow on the whole thing! I'd love a final season that plays like a horror movie.