Lehi's prophecies of his seed clearly show the coming agonies. His blessing to Joseph is not that his descendants will be spared the agonies, but that they should not be "utterly" destroyed. Of course this leaves room for a partial destruction, but the promise is that the lineage will continue. Thus regardless of how we understand the final demise of the Nephites (into which the Josephites had been at least politically subsumed) there would be physical descendants of that tribe, even when the political entity was long gone.