Who can tell what our species' journey will be. I honestly don't think homo sapiens will exist in 5 billion years, as that implies no evolution, no changing environment, no competition etc. And 5 billion years is a rather long time. Who knows what we'll learn, discover, be able to do between now and then. But the idea of migrating off-world is no different than people choosing to migrate to a different, untamed land called the Americas, or Australia etc. People choose to explore, to live on the limit, to push the boundaries. We scale mountains simply because they are there to be climbed. We walk to the poles simply because it is a challenge to do so. So it is that people will choose to live in a harsher environment than on Earth, because of the challenge, to be the first. For some it may be the only escape from an overpopulated world in which they find they have no prospect of being important, so they go to a small colony to help build infrastructure, knowing it will have a purpose, and thereby give them purpose. Who knows. But build it, and they will come. So to speak.