I'm not even a fan of Sanderson's writing and am amazed that Martin thought he had any latitude to be teasing and taking shots at another author's personal beliefs and professional work like that, in a public setting and to his face, while Sanderson is purely respectful in contrast. It's just so randomly and needlessly mean-spirited. It's like George never got his chance to be a bully when he was a kid, and now that he has just enough crest to his fame left to ride, he doesn't want to miss any more opportunities to pick on someone. Hard to feel that a little public humiliation in turn is undeserved.
He's basically one of those young, 2010s skeptics in an old man's body. Attacking another man's faith, taking advantage of his fellow author's professional nature, being mean-spirited towards religion, having low-IQ takes such as denying the existence of Jesus Christ, a fact that was well proven to the world, it goes to show that the supposed genius of GRRM is just the fans attributing smart things to his gibberish. If anything, the show did a better job of conveying the story due to the actors' charisma and being serious about their roles.
If you are aware of Cushing Library notes, then it should be clear that the show ending is likely to be based on GRRM's outline more than we suspect. It's very his style to just write down endings for characters without any explanation of how they went there.
For example: In one scrapped chapter found from his notes, it went for a few pages until the next one just said "Victarion dies" and that was the end of the chapter. If he gave similar notes to D&D (Bran becomes King; Jon kills Dany, etc) with zero explanation then no wonder it looked like such a cluster fuck. Dany burning KL is definitely from GRRM, however since the fat fuck didn't elaborate, D&D simply decided to make it looks like this. Similarly, there is literally no way that D&D made Bran the king of their own volition, they always hated magic.
That probably explains the delay in the books. The S8 ending was done in the spirit of GRRM'S writing, and the people hated it. It was the canary in the coal mine for Martin, and the poor reception to it caused him to rethink how he would end the novels. Throngs of book fans proclaiming that GRRM would never end the books like that has GRRM on edge; if they realized that it was his planned ending for the books, he will lose whatever clout he has with them left.
Dumb and Dumber have numerous flaws but it can be at least said that as long as they had the material, they did adapt it "relatively" well. It was only when they run out of GRRM's books that they decided to do whatever the fuck they wanted as Martin likely didn't even bother to explain his plans for most of the characters (probably because he didn't have any yet).
-D&D: What's your plan with Blackfish?
-GRRM: Dunno yet lol
-D&D: Okay, we'll just kill him then
One could argue that they still had some material from the Feast and the Dance, however both of these books mostly just needlessly introduced many more characters without tying any of the knots, so no wonder they decided to fuck it and just cut it off. To make things worse, GRRM is a lazy fuck and stopped contributing to the show after Season 4 and I doubt he kept much contact with D&D afterwards. He more or less expects others to figure out his genius master plan by themselves (and D&D were never that smart to begin with) while he's busy flying around the world and enjoying his fame.
That was the excuse that many GRRM book fans had. The idea that there were still more books and DnD had enough material for ten seasons. GRRM said something similar. However, most of that material is the literary equivalent of anime filler, and has little to do with the main story. If they were to be adapted, people would just get lost and lose interest. God knows enough anime fans dropped off their series of choice when it gives them way too much filler. Just imagine TV show fans.
At least, the way the show ended, it was quick and memorable, which is better than a prolonged death through people losing interest at a show filled with filler. And with GRRM not finishing the books, DnD would've eventually run out of material anyways. So GRRM's "genius writing" would've been revealed as the overrated stretch that it was, with plot holes big enough to fly a dragon through.
GRRM was barely even known of until Game of Thrones exploded in popularity and he suddenly became hailed as the American Tolkien. He tried his whole life to became famous and only became one in his sixties, as an old, done man. He has a mindset similar to Rekieta: he feels that he wasted his youth and now wants to indulge all his vices that he couldn't do when he was younger.
He's not emotionally or intellectually mature enough to be the next Tolkien. He's more a mix of a political thriller author with a strong influence from people like JJ Abrams and that whole "mystery box" nonsense. And along with that comes a lot of plot holes that logical people wouldn't have left unfilled.
Like, why would Littlefinger advise Ned against starting a war with the Lannisters if he wants the War of the Five Kings to kick off? If Ned listened to him, there would be no war, just the Lannisters and the Starks ruling and silencing any dissent.
Why would Jaime Lannister not tell Tywin and Robert about the Wildfire nuke that Aerys II was about to set off under the city, especially since it could still be there? Tywin and Robert would've believed him, especially since Robert hates Aerys and Tywin knows how crazy Aerys was.
Why did Tywin kill off Rhaegar's kids when putting them on the throne and marrying them to his children would've better served his cause, especially with the Rebel army tired after fighting in the Trident and the Lannister/Reach armies still fresh and ready for war? Between the two of those kingdoms, they'd have enough men to repel the rebel forces, especially since fighting Rhaegar and the Dornish would've taxed them to the limit. Tywin could secure a stronger position by having Cersei or Jaime marry Rhaegar's kids with Elia Martell, and in the process, gain Dorne as an ally for making Elia Martell the Dowager Queen.
All of these plot holes show me that GRRM is not the master storyteller that the book fans proclaim him to be. He struck gold with the first three books and the background materials, but the latter two books, while having some good ideas, got so bogged down in plot that trying to tie it all up is a herculean task I wouldn't wish on my worst foe. And no matter what ending GRRM opts for, it won't be the big, cathartic payoff that his fans were waiting for. More like something similar to the show, just boring and more convoluted. Especially when Martin isn't a big believer in monarchy or religion, so don't expect him to end it with a good ruler sitting in the throne after completing some prophecy.