I’d be really interested in reading a book or two covering the events immediately after the end of A Dance With Dragons, but alas, we probably never will.
My theory is that he was already well known but the show made him actually famous. He got caught up in celebrity life and his writing slowed down even more, and then, the shows ending really was his ending. And everyone thought it was fucking stupid. So he stopped writing. There will never be another main sequence book.
I think you're mostly right - afterall the ending being the same as GRRMs is basically confirmed, and ultimately the basic building blocks of the show ending could be made into a satisfying ending if they weren't so damn rushed and if they had better motivations.
I do think he is still writing though, I just think the fire has gone out. He just keeps writing drafts, trashing them, starting over, repeat. I will be shocked if we ever see TWoW, and I'll eat my shoe if we ever see aDoS. And frankly if we do see TWoW, I think it'll be a let down, because it's clear that GRRM is mentally checked out of the story now