1) The Book of Mormon itself never claims to be a record of the entire inhabitants of the entire
2) We have a very spotty record of one family, its descendants and those with whom a portion of that family came in contact. There is almost nothing that gives any information about others who might have lived on the continents - except an even spottier record that followed the Jaredite royal line (or, more probably, those who remained close enough to the “capital” to remain under the control of that royal line). When you read the Book of Ether, it is obvious that it does not even try to keep track of all the descendants of the initial group. It focuses almost exclusively on the kings and what happens to them, only including the people around them as fodder for the tales of warfare and intrigue. In other words, after the initial explanation of the founding journey of Jared and his brother, almost the entire book is a politico-military treatise - not primarily even a religious or spiritual history of an entire people.
3) “All the land” is a common term used in ancient scripture to mean “all the known land” or “all the land under centralized rule”. Noah’s flood is seen by many Biblical scholars as having been a catastrophic local flood that wiped out everyone and everything that inhabited Noah’s world - one that was later generalized to universal status since every culture has a similar flood narrative - since nearly all large civilizations anciently were centered near and around water. (Also, a careful reading of the chapters immediately following the flood make it crystal clear that there were other human survivors of the flood outside the flood area.)
To highlight this tendency, Caesar’s edict that “all the world should be taxed” is obvious hyperbole; it simply meant “all his world” - since he was well aware of areas that were not under his control and, therefore, would not be taxed. Even just the Jaredite descendants who stayed close to the capital city easily could have numbered in the millions; they easily could have had cities that stretched from “sea to sea” (especially if the central government was located somewhere - anywhere - where "sea to sea" could have meant "huge body of water to huge body of water"); and those millions that lived in that area easily could have been rounded up and destroyed in a final, massive battle - all the while not coming close to wiping out every living human on the face of the American continents other than the Nephites and Lamanites and Mulekites of the time.
Again, this is based on what the Book of Mormon actually says about them, not culturally-based assumptions made by those who read it after it was published. Coriantumr easily could have been the sole survivor of that centrally controlled empire, while millions of other descendants of the ancient Jaredite group could have been living outside the control and knowledge of the royal family. Basic population and demographic standards almost dictate that such would have been the case after what probably was well over 2,000 years. [In fact, the incontrovertible fact that the Jaredites were unaware of the Nephite/Lamanite/Mulekite populations living relatively close to them (at least within walking distance for Coriantumr at the time of the great and final battle) argues that they had no clue whatsoever if “all” the people had been gathered, but rather gathered only those of whom they were aware.]
4) If the descendants of Lehi were a localized group, covering a relatively small area (compared to the entire continents), and if the “Lamanites” assimilated other, larger groups not specified in the Book of Mormon - like the Nephites assimilated the more numerous Mulekites (which appears almost certain, given the population descriptions included in the Book of Mormon), then their genetic footprint easily could have been diluted very, very early on.
5) Furthermore, we have no idea whatsoever what the maternal lineage was for Lehi’s children, since we have no idea whatsoever of Sariah’s lineage. Lehi appears to have been a traveling man who was outside the accepted authority structure of
In summary, there are so many assumptions that underlie our beliefs about the Book of Mormon that sorting out what the book actually says from things that simply are assumptions ("incorrect traditions of our fathers") is vitally important in this modern day of discovery. I have found that the more I understand what it actually says, the less I am bothered by new discoveries - and the more I am amazed at the accuracy of the book itself.