Imaculata
Hero
Neither. He just wrote himself into a corner by setting out to subvert fantasy tropes. One of those tropes is "has a satisfying conclusion". In real life, stories never end - something Martin's archenemy Tolkien acknowledged.
The TV version had this problem. You can't subvert "has a satisfying conclusion" and still have a satisfying conclusion. Martin's solution is cleverer - die before finishing the last book, so there is no conclusion.
I don't think that his intension, but it might be what ends up happening. The show suffered from no longer relying on Martins writing, so they wrote their own disappointing finale that rang false of the show and book's identity.
Has Martin written himself into a corner? I don't think so. But I do think he is more distracted by all the side projects he has going on. I think the success of GOT has become a burden for him, and he may have lost the love to finish his own creation. Some may call that 'selling out'. I just call it writers fatigue.
I think subverting tropes does not mean you can't have a satisfying conclusion. But it seems a happy ending does not fit this series. I think the reason that the new book isn't out yet, isn't a problem with writing, but a problem with wanting to write. If your heart's not in it, the ideas don't come either.