Indeed there are people who don't contribute to the community. Or maybe they can't find a publisher. I'm sure there are brilliant things written which will never find an audience.

But the broader point, in agreement with the article, is that the existence of fiction depends in part on extrinsic motivation (e.g. publishing, finding readers) and a community of intellectuals engaging with one another through their work. If everyone decided to stop seeking readers all of a sudden, the field would die and become a museum (mausoleum?) of former thinkers.

Contributing to the community posthumously still counts as contributing.

Kafka did publish some of his work during his lifetime, including The Metamorphosis.

Yes, but much of it wasn’t, which was my point, as it’s simply the first case that comes to mind where we know an author demanded their unpublished work be destroyed (Terry Pratchett is another). And Kafka didn’t receive widespread recognition until after his death, when Brod started publishing his writings en masse.

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 Also, we should be careful about interpreting his wish to destroy his unpublished works in a certain way; much of it was unfinished, and he was kind of a perfectionist.