Luck. Model an agent that has some odds of doubling wealth, maintaining wealth, or halving wealth per iteration of some game. It's easy to generate downward trends.
Yes. TFA ignores what I think is the biggest factor. In researching family history, I've been struck by how important chance was regardless of how smart and hardworking they were.
Yeah it was a bit annoying they didn't mention luck in the opening of this article. That being said, work ethic and a some inoculation against nihilism are necessary ingredients.