Mormon includes the three groups of people who would be involved with the Book of Mormon in the last days: the Old World Jews, the New World house of Jacob, and the believing Gentiles (who bring the gospel to the remnant of the house of Jacob). These are the three groups of people involved in the Savior’s prophecy as recorded in 3 Nephi 21.
Mormon also repeats his traditional cultural position contrasting Lamanite/Nephite as bad/good. The righteous who are “white and delightsome” are becoming unrighteous, or “dark and loathsome.” There is no missing the intentional contrast. They describe opposite poles of righteousness—not skin pigment. (See commentary accompanying 2 Nephi 5:21.)
Mormon acknowledges, sorrowing, that his people will survive until the Book of Mormon’s coming forth by becoming in every way like their Lamanite neighbors. They will adopt all of Lamanite culture, including “their unbelief and idolatry.” This statement also indicates how Mormon’s Nephites have already become like their enemies.
Mormon sees the depths of his descendants’ degeneration as “beyond the description of that which ever hath been amongst us.” In Mormon’s own context, warfare has developed two appalling innovations. The conquering Lamanite/ Gadiantons are burning enemy cities and also sacrificing women and children. As I read Mormon’s record, he is witnessing the transformation of his culture under the influence of the invading Teotihuacanos and seeing his descendants’ world in their new methods.