Physics teaches us that in any closed system, entropy, or disorder, will always increase. Put another way, groups will gradually become indistinguishable from each other, given enough time. They will grow, interact, and merge, with individual behavior becoming more random.
As beings who value order, this fate is unacceptable. There are two ways humanity has dealt with this problem. One way corresponds to the retreat option in attaining security: Exploring regions untouched by people, or effectively “opening” the system (available habitat). The other way limits the size of the population so that more of the system is available to those who survive.
In practical terms, “opening the system” would involve exploring and settling other planets. This is the most humane approach, effectively postponing the inevitable encounter with the ultimate limit to growth: our inability to reach new resources fast enough.
The alternative may be a series of paroxysms of growth and death as the population reaches a maximum, declines to a level where people are able to express their uniqueness and rises again.