It’s very obvious that he’s writing Winter and Spring at the same time and planning a duel-release of some sort. I know you’ll all laugh, but I genuinely believe he’s doing it.
It’s taking him longer because he’s hoping to vastly improve on the show’s ending. He’s vain enough to make everybody wait for it too. He wants his magnum opus to be the literary crown of the 21st century, and an infamous duel-release will secure his legacy.

You’ll all see.

Did this come to you during a dream in a spring afternoon or a winter's evening. George Martin is 76 years old. He is portly and wealthy and no doubt inundated in recognition and adoration after a youth of invisibility. His calendar is no doubt peppered by hundreds of little interviews and appearances and meetings and meetings and for even a healthy 76 year old this would consume all of your energy. But we all know healthy he is not. The mere thought of confronting his totemic DOS PC turns his formidable belly. Success has never been known to make the creation of great art easier. His life must be garbed in comforts and pleasures that make the suffering if writing massive bundled novels impossibly daunting.