If you were paying him in ongoing installments as part of a contract with a specified deadline, then yes. But you're not. You haven't paid him anything yet for all of the time he put into A Dance With Dragons, and you won't until you buy it. You paid him for the time he put into the other books. When you bought A Game of Thrones, you were paying him for that book only, and for the time it took him to write it. Not a penny of that was for anything else. He could stop writing the series tomorrow, put it down forever, and there would be no recourse. None. Because you haven't paid him anything for future books. The money you pay for his books is not an advance on the next book. It's for the book you're buying, period.

Being upset with him for taking awhile is a valid enough emotion, even if I personally disagree. Refusing to buy more of his books is an option you have. Does Martin have something of a moral obligation to finish his series? Sure. But a financial obligation? A legal obligation? No. A thousand times no. His life is his own. You don't own or possess any part of it, nor any part of his future time. Please stop pretending you do.

If you think that somehow you've already paid for his future services as a writer, then why do you have to pay again when you buy Dance With Dragons? Why didn't you have to pay for Game of Thrones before he started writing it?

Think of a serial story, the old kind that came out in newspapers. Let's say you buy the first three newspapers that have the first three parts. When you go to buy the fourth, the news stand is fresh out. Does anyone have any sort of obligation to provide you with that paper? No, you haven't paid for it yet.

From a different angle, say the author of the serial story stopped writing it, or the newspaper decides it wasn't popular enough and stops buying it from the author. Does anyone have an obligation to give you part four? No, you haven't paid for it yet. You bought the other three parts readily enough, even though you knew it wasn't finished yet, and that's the risk you took. You felt it was worth it because those first three parts were really good. You want to know what happens next, but this is how the story is released, and you haven't paid for the next part, so you don't get it.

Be as angry as you like at how long it's taken, but there's no binding obligation. And trying to enforce "reasonable" time limits on authors who are telling a story is one of the weirdest things I've heard in a long time. Maybe writing works like that for some people, but I don't know any of them. Even Brandon Sanderson writes because he loves it and wants to tell stories, not because of some obligation. He writes fast because that's how he writes. If he didn't write that way, then he wouldn't. End of story.

"It's good business." Isn't that his concern, not yours? Can't you run your business any way you like?