In Words of Mormon 1:2, Mormon makes a very interesting statement:
And it is many hundred years after the coming of Christ that I deliver these records into the hands of my son; and it supposeth me that he will witness the entire destruction of my people. But may God grant that he may survive them, that he may write somewhat concerning them, and somewhat concerning Christ, that perhaps some day it may profit them.
Hugh Nibley poses the question, If they [the Nephites] were all destroyed, how can it "profit them" someday? The answer is that Mormon doesn't mean entire destruction. There were survivors, as far as that goes. But the word destruo means to break down the structure of the society, that sort of thing. The same can be applied to the destruction of the Jaredite civilization. [Hugh W. Nibley, Teachings of the Book of Mormon, Semester 1, p. 435]