Rose and Bernard were, I'd guess, at least 65 years old when the plane crashed. If they were sent back to 1974, by 2004 (when the bones of Adam and Eve were found in the cave) they would have been at least 95.
Therefore, there's a good chance they died of old age in the past. Which really, given how happy they were on the island, isn't a bad way to end their story.
