I am a Big fan of George R. R. Martin's fantasy book series A Song of Ice and Fire and the show on which it was based (Game of Thrones) before it turned to shit, that is, at the hands of Dumb and Dumber. Although a lot of people like to blame D&D for ruining the show and rightly so, here's why I think GRRM is going to be incapable of finishing his book series with still two books to go for the conclusions.
-First: GRRM lives in a liberal echo chamber / swamp. It's become clear now that no one is safe from cancel culture. Apart from being pilloried for continuing his book series faithfully and as originally intended because everything is x-ist now. I think this goes deeper than just online fake backlash. In my estimation and maybe i need to do more digging to prove this, that most of not all of GRRM's personal relationships are with people who are overwhelmingly liberal that he would naturally fear being ostracised from his friends, family and acquaintences for doing what he has been already doing for ages. Cancel culture isn't online anymore it's a way of life. Not even J.K. Rowling was safe. But that's just the first and most superficial reason.
-Second: GRRM has lost touch with the muses. ASOIAF has always been a literary artwork that has always been in touch with a deeper reality. It's heavy use of symbolism and imagery echoes many religious stories and mythology, similar to Lord of the Rings or many other great works of art. Good art is good because it opens a window into the deeper realities and mysteries of the human condition. Call it spirituality, collective unconscious or anything else. I do happen to believe in a certain idea that the artist is merely the tool or executor of that mysterious force we call inspiration. A good artist regardless of their personal beliefs is capable of producing great works of universal values if they merely do the work that the muses channel through them. Once ego gets in the way the artist is blocked and unable to produce or continues to produce but it's garbage. I think GRRM has lost his connection with the deeper mysteries, i can only speculate why, but i happen to think his ego, fame, fortune, politics and the people around him might be factors in this.
-Third: He doesn't know how it will End. I happen to believe in another certain set of ideas which I learned from Jordan B. Peterson which posit that artists are usually spiritually sensitive neurotics. They see the world around them and draw inspiration from it and by allegory or other artistic tools communicate something that is not obvious at it's face value but with immense spiritual importance to understanding the current Zeitgeist. Starting his series in the 90s i do believe GRRM was glimpsing unconsciously through a lot of the bullshit and whether he knew it or not was giving his readers a tool of understanding the world we live in today differently, think of it as a subtle dose of red pills administered by analogy, familiarizing his audience with the brutality and wonder if the human condition. He doesn't know how his story is going to end because he doesn't know how the ongoing spiritual battle of our time is going to end. In a way no one can be sure. Will we lose everything and become slaves to the new world order? Great reset? Great awakening? Mix of both? A completely new paradigm no one saw coming? Anything could happen and we do have faith in certain things such as Qanon or Trump's hopefully inevitable victory but so many things can happen in the chaos that there's no 100% certainty in life. GRRM losing touch with the deeper spiritual context means he is incapable of mustering the clairvoyance to finish his story in a way that will satisfy him or his fans.
-Fourth: Sabotage. This is probably the most far fetched reason and I don't even know if I do believe it but I'll throw it out there anyway. A work of spiritual significance that can redpill and inspire millions is surely to be the target if the kabal in their ongoing March to dominate culture and art. Nothing was safe from them: Star wars, Star Trek, doctor who, and countless other works of art. Why should ASOIAF be any different. He might have been coerced: blackmailed or bribed to grind his work to a halt, or he might have just been corrupted with his success in hollywood (my buddy calls it the hollywood effect). In any case I out forward the theory that GRRM and his work were targeted for elimination by the evil forces in the world precisely because of the spiritual significance of ASOIAF.
I do think GRRM is one of the great authors of our time, and generally hope that my thoery is wrong and that he finishes his story faithfully, but at this point he might be a victim of the spiritual disease and battle that is taking place in our culture and our hearts and minds individually. He will either die without finishing it (and we can hope for posthumous releases of unfinished works) or he wont be able to stick the landing and it will be another series ruined.
I think it's simpler than this. He started writing the first book in 1991, the beginning of an optimistic decade when a grim dark fantasy series would be contrarian and hit hard. In 2020, the real world has become much darker than it was in 1991, and nobody wants to read or write a grim fantasy story. He no longer feels badass for writing a dark fantasy during an optimistic time, so he doesn't write it! Another manifestation of this phenomenon is Charlie Brooker stopping work on Black Mirror. Brooker explicitly said that the world has become so dystopian that he no longer feels motivated to write dystopian fiction.
I enjoyed the ASOIAF series but it's not really in the same league as, say, LotR in my opinion. It's like an edgelord discount version of LotR. With a bunch of sex and violence just for the sake of making it "gritty" and "dark" but really there's nothing special or new about the series that hasn't already been done to death in the (medieval) fantasy genre. That said, I agree with you that GRRM is probably never going to finish the series. I doubt he's even trying at this point, he sure seems unmotivated about it. Plus he's a fat fuck and probably one ill-placed cholesterol molecule away from a fatal heart attack or a stroke.
GRRM is a B tier fantasy author and an S tier author like Joe Abercrombie or Robin Hobb will be given permission to finish the series by his estate.
I think it's even simpler. He lost his crew. Daniel Abraham and Ty Franck left to write 'The Expanse' series. GRRM obviously can't finish on his own.
It would be cool if all the characters that have narrated the chapters die before the end. So no one knows how it ends because all the people telling the story died before seeing it through.