Sir Terry was a perfectionist in his work and each book was polished until each word shone, reading the Shepherds Crown you could see how unfinished, to his standards it was. This isn't a criticism of the story but a fact of his illnesses effect.
For anyone else to finish his books would be wrong and it was right to follow his wishes, although to publish his unfinished work as it stood would have been a fascinating insight into the way he wrote.