Jacob notes that the Lamanites are faithful in keeping this commandment. He leaves unstated the assumption that they have violated many others, but nevertheless depends upon that understanding to highlight the crime of the Nephites in disobeying a commandment that the Lamanites obey. Jacob also parallels the threat of destruction to the Nephites with the promise of salvation for the Lamanites. As noted, this situation does occur within the next several generations, with the Nephites being greatly destroyed with a remnant moving to Zarahemla, and the Lamanites remained in the land, and prospered. The fulfillment of the Lamanites becoming a blessed people may be reserved for the last days, but my also simply refer to their success in the land.
One should not necessarily read into this very specific promise a wholesale promise of destruction for a people living polygamy, as we have already discussed the very specific temporal nature of this commandment to the Lehites. Similarly, the nature of the fulfillment of this prophecy suggests that it, too, was tied to a specific time and place.