Verses 30 and 31 are a contrast to verse 30.
There is joy for this generation where none are lost.
There is sorrow for the future generation where all are lost.
This generation has been purged in the fires of the coming of the Atoning Messiah. They have seen and believed, and their faith is strong, and will continue to be strong.
The Savior prophesies that this current faithfulness will fail in four generations. Those future descendants of this faithful people will be led away. The imagery used for the failure of that generation again refers to the text of the Sermon (3 Nephi 13:19). In particular, the Savior describes the reason for the eventual apostasy, and it will be the same problem that has plagued the Nephites all along. They will desire the treasures of the world, and this world will pull them out of the treasures of the next.