The purpose of prophesying future devastation is not to change the future, but to influence the present. Thus, while the picture Jacob painted showed the result of unrighteousness, he now turns to the positive side of the equation. There is repentance.
Upon repentance, Israel can be restored to righteousness. The scattered Israel in Jerusalem can be gathered. At this point, Jacob returns to the text of Isaiah 49:23 which Nephi had given him as a beginning text and with which he began in 2 Nephi 6:7. In the early case, Jacob appears to have used that text to speak to the value of the gentiles in the midst of the Nephites. Here, Jacob returns to Isaiah’s original context, which was the future of Old World Israel.
In this way, Jacob has tied Isaiah’s prophecy to both the Old World branch of Israel and the New World branch. He sets them up as parallel, and therefore parallel in the gathering of the righteous branch (with which he began this second day’s sermon).