Question: How can Moroni be the last survivor of the Nephites when modern revelation reveals that Nephites are living in the latter days?
Answer: The answer depends on the usage of the word Nephite. If the definition refers to descendants of Nephi, then Moroni was not the last survivor. Doctrine and Covenants 3:16-19 refers to Nephites living in the latter days. (The Book of Mormon comes forth to save the seed of Lehi)
Hugh Nibley asks, “Are there not many Latter-day Saints who will insist that every American of pre-Columbian descent must be a Lamanite because...the Nephites were destroyed? Yet the Book of Mormon itself makes such an interpretation impossible...Only once in the Book of Mormon do we read of a case of annihilation, when we are specifically told that every living soul of the Ammonihahites was destroyed (Alma 16:9), where not only the social structure but each individual is undone.”
If the definition refers to followers of Christ, then, as far as the Book of Mormon record is concerned, Moroni was the last survivor.
Source: 400 Questions and Answers About the Book of Mormon by Susan Easton Black, p. 197.