Hm, but I'm not sure I understand your point. Are you saying that Kafka is a counterexample to the need for external motivation? It's true that some of his best-known work was only published only after his death (which was against his wishes, though Brod had supposedly told him that he would not destroy his works). But he did still have external motivation, e.g. Brod encouraged him, and Kafka read aloud some of his work to his friends. Also, we should be careful about interpreting his wish to destroy his unpublished works in a certain way; much of it was unfinished, and he was kind of a perfectionist.