I dont think you understand how this works
HBO isn't likely to sit on their hands waiting however long for GRRM finish the series. They're a business and they intend to keep some level of consistency in their programming. What GRRM and the rest are hoping is that his books are written in a time span that allows for them to be ahead of the show. But that is hardly a guarantee, and it shouldn't surprise anybody if they lag behind. GRRM can fairly easily tell them what happens, and already has to some degree, and they'll just adapt that.