It is said, that the isu are "forgotten" (spoiler: after the destroying of Atlantis in Odyssey), and the humans will just name them again, stop worshiping them and continue living as the old ones die. Same logic in Valhalla, as the Åsgard is destroyed and the isu die/are thought dead, the humans remember them in legends and stories, and these stories tend to change over time.
Even the christian god is mentioned by the Order to probably have been an isu, or at least something else before, just modified by humans to support their beliefs.