Exodus Imperative (Civ6)/Civilopedia

Space exploration is a fairly compelling subject for many people, but the idea that human beings must make it a collective priority to leave the Earth and settle planets is more of a trope of science fiction than a commonly-held belief. But if it were, this belief would be the Exodus Imperative.
Taken broadly, the idea is that humanity will fail to achieve its potential (and may risk extinction) if it does not make the effort to move offworld and settle other planets. Various dire interpretations of what might happen if we do not leave Earth have been offered: Conflict over increasingly scarce resources, destruction at the hands of a planet-wide catastrophe, and so forth.
Certainly going Beyond Earth is a fascinating idea, and offers a student of speculative fiction a number of interesting ideas to explore. What would be relationship be between those who remain behind on Earth and those who leave it? Could humanity live in harmony with any alien life forms it encounters? How would developing attitudes towards these questions affect the relationship between groups of colonists? These ideas are clearly fascinating and worth exploring in detail elsewhere.