Not all the Nephites were killed at Cumorah. There were four groups which survived, at least temporarily: 1) the twenty-four survivors of the final battle, 2) the group who had tried a southward escape (Mormon 6:15), 3) those who had deserted to the Lamanites (Mormon 6:15), and 4) the robbers which may have been of mixed Nephite, Lamanite lineage (Mormon 8:9). Except Moroni, the members of the first and second groups were eventually hunted until they were killed. But the third and fourth groups were never completely exterminated. The Nephite deserters and Gadianton members survived. However, they did not retain their identity but became Lamanites, as Alma prophesied. Hence, the Nephites, as a nation and as a people, had been destroyed.
The fact that there was considerable mixing of Nephite blood and Lamanite blood is significant because of prophecies which describe the descendants of Nephi in the latter-days. Nephi was told, God will not suffer that the Gentiles will utterly destroy the mixture of thy seed, which are among thy brethren (1 Nephi 13:30, italics added). Through modern revelation, we know that some of the blood of Nephi, Joseph, Jacob, and Zoram was preserved among the Lamanites. Nevertheless, my work shall go forth…even so shall the knowledge of a Savior come unto my people—And to the Nephites, and the Jacobites, and the Josephites, and the Zoramites, through the testimony of their fathers (DC 3:16-17).