Jacob’s message isn’t that the destruction will come, although that is what will occur at the end of the purpose of the earth. The message is one of repentance to avoid a more present possibility of destruction. Throughout the allegory the Lord of the vineyard cares for his olive trees, even when they have had problems, even when some fruit was corrupt. That message of the overarching concern Jehovah has for his people extends to the Nephites, and allows them the possibility of repentance.
Repentance is the message. Therefore, Jacob declares: “I beseech of you in words of soberness that ye would repent, and come with full purpose of heart, and cleave unto God as he cleaveth unto you.” That Lord who cared for the vineyard, cares for his people.
Most poignantly, Jacob asks: “today, if ye will hear his voice, harden not your hearts; for why will ye die?” The call to repentance is immediate, and the promise of the land is that they will prosper only upon righteousness. The result of unrighteousness is destruction. The question “why will ye die?” has two meanings. In the most obvious, it refers to a physical death that would be the result of the destruction of the unrighteous. However, it is also a spiritual death that is the direct result of turning away from God.