Martin's editor is among those who think it's important that he stay ahead of the show and publish his ending before the show reveals it:
From what you’ve read about The Winds of Winter so far, and, without spoiling anything, how good do you think the book is compared to the others in the series?
It’s amazing. In fact, when I first read the partial back in 2013, I immediately wrote George an impassioned plea to please not let the show get out ahead of him. I can’t predict what will happen from here, but I definitely want his version to be my first version. And I think a lot of you want that, too. His vision started this; I very much want his vision to end it, too.
I think it would be great. But, I would rather he writes the book that he wants to write than cut corners just to get it out "in time."
He's not dumb, he knew it was a possibility.