The whole subplot is just a small comment. The movie stands well on its own, but that subplot further solidifies a smaller theme many people may not have picked up on without: the experienced and the leaders didn't really give a shit about the consequences.

The dog was meant to ground their perspectives.

The guy knowingly participates in the fire sale and subsequent economic collapse of the United States, but it still doesn't hang over his conscience more than his dead dog.

The movie ends with him burying his dog. At the end of the day, the death of his dog was more important to him than what he's done to the economy.