You're right, I'm not giving GRRM the ire he deserves here.
We don't know the behind-the-scenes on this; maybe D&D lost the support they had which made the show a success early on (from GRRM), and decided to wrap it up. IMO though they're all 3 of them in the muck on this. So agreed there.
Honestly at this point I'm glad I never bothered to read all the books that have been printed, as I didn't enjoy his writing in the first place. I loved the show but found the book to be poor compared to other fantasy I've read, e.g. Brandon Sanderson (even with all his Mary Sues).