The ending is there though, he just has some of mental block going on there. Still I don't think that excuses D&D, they wanted to bail out but still wanted the credit of seeing the series through to the end.
It was impossible to have both. You either stay and do it right or you leave and let someone else do it properly.
There's way too much story given to them by GRRM to properly and no where enough time to properly tell that story in just two shortened seasons. Not based on the first 6 seasons. You can't just radically change the pace of a show in season 7.
He doesn't have a mental block, he's producing other stuff because he doesn't know how to finish ASOIAF.
He's constantly talked on his blog about how he has had to rewrite huge chunks of his manuscript to accomodate minor changes in events.