The scene moves to the New World and to the Lamanites and the Nephites. Both parts of the original branch planted in this land have now become corrupted. One had become corrupt earlier, but not both of them had. The earlier state showed the relationship between Lamanites and Nephites while the Nephites were righteous. However, the Nephites did not continue in righteousness. Although the allegory doesn’t know of the promise of the land for the Nephites, it does depict the fulfillment of the negative aspect of that promise.
The Nephites did not continue in righteousness—therefore they were destroyed. In the allegory, they are the branch that withered away and died.