I get the feeling that they're overthinking this a bit. They're really looking at the possibility that we're ALL completely dicimated and that an alien culture is coming down to us, because otherwise, there's a way for stories to be carried on. Even as we look back at the caveman paintings, ancient egyptian scripts, we can decipher a lot, though their cultures are so different from ours... so lost. A culture with the capability of traveling to a desolate, decimated earth will likely also have the ability to figure things out like language, and will hopefully have the kind of fore-thought required to determine whether an area is safe, or at least not jump the gun. If they're not such a kind, thoughtful culture, perhaps it's best that they jump right into the radiation, know what I mean?
