I would agree with you if each book was a stand alone. However, he's writing the story as a series of books. A writer who doesn't finish the story is not a good writer. I think ultimately we'll be able to decide whether the series as a whole is good is only when the series is finished.
Looking at the individual books, the first few we excellent and why the series has the good reputation that it has, but they had something in common, they each had a beginning, middle and end within the context of the overall arc. Where I think things have gone downhill is that he hasn't yet finished the book that started in Feast and continued in Dance. Originally this was intended as one book. First he split it into into Feast and Dance, and then when the TV series was launched, he had to deliver Dance so that it could be published that summer. He hadn't finished it in time, so had to leave off the ending deferring it to The Winds of Winter. So one could argue that he's not actually published a finished book in the series for fifteen years and counting.