These words are what Mosiah wrote, not what Mormon abridged. His words continue through verse thirty-three.
The “descendants of Nephi” (Mosiah 25:13) were the ones to whom the kingdom rightly belonged (29:6). There is no record of when or by whom this patriarchal line of kingship was declared, but it originated with the Lord’s promise to Nephi that he would “be made a ruler and a teacher over thy brethren” (1 Nephi 2:22). An angel of the Lord later reminded Laman and Lemuel that “the Lord hath chosen him to be a ruler over you” (1 Nephi 3:29). Nephi had no desire to be his people’s king but did fill the role because of what the Lord had told him (see 2 Nephi 5:18–19). Those who did “reign in his place and stead were called by the people, second Nephi, third Nephi, and so on according to the reign of the kings” (Jacob 1:11). The record does not say whether his successors were or were not his sons or direct descendants.
Jesus Christ will reign as king in the millennium, and the political right to rule belongs to the tribe of Judah through which he was born. The Lord promised David, king of Israel, through the prophet Nathan “And thine house and thy kingdom shall be established for ever before thee: and thy throne shall be established for ever” (2 Samuel 7:16). Christ will be the ultimate fulfillment of that prophecy. The Prophet Joseph Smith said: “the throne and kingdom of David” will be “given to another by the name of David in the last days, raised up out of his lineage” (TPJS, 339). This often controversial prophecy will also be fulfilled by Jesus Christ in the millennium.
King Mosiah’s, son of Benjamin, explanation of why another should not be appointed in his son’s stead is well reasoned and self-explanatory. His final statement of these verses gives the basic purpose of government, and correlates well with Doctrine and Covenants 134:2. Mosiah, son of Benjamin, says the system of judges would bring peace to his people (Mosiah 29:10). Paul taught the same concept: “For kings, and for all that are in authority; that we may lead a quiet and peaceable life in all godliness and honesty” (1 Timothy 2:2).