Mormon’s mind’s eye is looking to the future. He began with the coming forth of his text, but since his focus was on his people, he returns to the future conditions of those people. The destruction of the Nephites and the scattering by the Gentiles is an old prophecy (most notably 1 Nephi 13). Mormon restates it here. The Nephites are currently being driven by the Lamanites. The mixed remnant will similarly be driven by the Gentiles. Mormon likely sees this coming entrance of the Gentiles as just such a parallel event, brought on the by the similar wickedness of the mixed descendants of his people who will survive until that time.