This is not simply a lecture to his sons, but it is a prayer for them. It is Lehi near the end of his life expressing his fears and desires for his sons (and daughters, though they are not explicitly addressed). In this he is like most fathers. Children are the focal point of so many hopes and fears.
For Lehi, he has reason to fear. Prophecy has indicated that some of his descendants will not follow Jehovah. Personal experience has taught him that Laman and Lemuel, in particular, were rebellious and in grave danger of declining to partake of the fruit of the Tree of Life. Thus, when Lehi fears that the hardness of their hearts would cause their destruction, he is referencing prophecy more than simple parental anxiety.
What is interesting is that fears of destruction are placed in the context of Laman and Lemuel’s rebellion; however, they are communicated to all of his sons. That is important to remember, because even though it is certain that Lehi and the early Nephites saw the prophecy of destruction applying to the Lamanites, it would be the Nephites who were eventually “visited by sword, and by famine,” and destroyed.
In spite of the prophecy, and in spite of the physical realities of the wars and famines, what concerned Lehi most was that they would be “cut off and destroyed forever.” Not that they would be cut off from the Old World, but that they would be cut off from Jehovah.
There is a solution, and Lehi repeats the promise of the land. They would prosper in this new land if they kept the commandments, but if they did not, they would be cut off from Jehovah’s presence. That is a more devastating and final destruction than the physical end of a people.