The early descendants of Adam and Eve lived longer so that they would repent while on earth.
The Zion people of all the universe will come to this earth in the city, the New Jerusalem, at the beginning of the millennium.
If Nephi took the Brass Plates of Laban to the promised land, how could the Old Testament scriptures have been available to the people in the Old World?
The Brass Plates of Laban may well have been the original scriptures. But many copies in the Hebrew langquage were available after Nephi took the Brass Plates to the promised land.
The fulness of the everlasting gospel is contained in the Bible because the Book of Mormon says so.