One of the themes for my novel is The Frontier. Without space to grow, a society has no release valve for it’s members that don’t fit in. Now, since progress mostly comes from those who do not ‘fit it’, a society with no frontier will stagnate. That’s what happens to Earth for a couple of centuries as they attempt to control the borders and travel. This, combined with longevity treatments for the elite and their pets, results in a stagnant fearful culture that quickly gets left behind by the people who leave during the Second Great Diaspora.
Cultures change, though, they always do. Earth eventually throws off the yoke of the Immortals, and begins to turn it’s eyes outward again. This brings a huge load of trouble to the younger cultures that have spread to the stars.