He may not finish it, but you can be damned sure the producers of the series have a solid plot line at their disposal should he kick the bucket.
True, but I don't expect them to have the same level of detail or intricacy the completed series would have.
It's quite a complicated world. It's easy to miss out on the little clues scattered here and there - such as Young Griff's possible ancestry; who was responsible for Balon's death; the creaking hinge of Aeron's memories; the identities of the Sphinx, the new Pirate King in the Stepstones, the brother on the Quiet Isle, Robert Strong, and more; or what actually happened at the Tower of Joy, etc.
And that's what we actually can claim to be pretty certain about, if you're a careful reader. There's so much unrevealed or left ambiguous - what is up in the Land of Always Winter, who is Septa Lemore or Coldhands, what happened to Benjen (and no, he's not Coldhands, they killed him "long ago"), what's on the Isle of Faces, what did Rhaegar read that lead him to become a warrior, where did Tysha go, etc. Now some of these won't be answered most likely, but we'll get more answers, both in the main series and in the Dunk & Egg novellas.
I think the television series can pull off a satisfactory conclusion, especially since it veers off on its own direction occasionally (Yara's plotline, for example) and by the necessity of the television medium it must be more simple than the books. But I don't think the written series will be the same if someone else finishes it.