1) There are no "self-sustaining colonies" any more. Even here in North America, the richest continent on earth, in both resources, skills and cash, we can't maintain our industrial and technological civilization without trade with other countries. We can't build a colony anywhere on earth (certainly not in space) without supplying it constantly from outside with raw materials and manufactured goods. And in a really harsh environment, like the arctic, or the oceanic islands, or the desert, or the rain forest, only stone-age societies can live off the land. All those environments were fully exploited by neolithic peoples, but we can't do it without massive technological and logistic support. Civilizations need trade and external supply, and a base in the asteroid belt,earth orbit, or any planet or satellite in the solar system, cannot exist without massive outside support. Permanent bases on Jupiter's moons are possible. Self- sustaining colonies, NFW! We tend to think technology can solve any problem, but it needs constant maintenance and propping up. The more tech you've got, the more support you need, and you can't live in or go to space without lots of tech.

2)Profit, slavery and missionary work is the main reason the New World was settled by Europeans. In North America, a few small scattered colonies were started by religious fanatics and misfits from the mother country, but this was an outlier. Even in N America, the primary reason people came here was for profit. Even the religious cults and sects and political rebels that came here fully expected to finance their holdings here with trade with the old world. As it turns out, growing dope (tobacco, sugar, cocoa, coffee, tea and spices all qualify for that description) quickly became the cash crop. Second was luxury goods, like furs, indigo, Aztec Silver and Inca Gold, and silks and ceramics from China.

3) The idea that space will allow us an escape valve for excess population just won't wash. The cost of shipping one person to a space destination and supporting him there until he is productive is prohibitive. Even in our own history, the people who settled the New World were only a tiny part of the population of the Old, And unlike what we were taught in high school history class, the Colonies were a hell-hole. Only the desperate came here, and they traded their freedom for a ticket. Remember the indentured servants?

3)There are a few historical examples that may be more illustrative of interstellar colonization. The Greek city-states of Classical antiquity sent colonists throughout the Mediterranean and even the Black Sea. Many of these survived to become prosperous cities themselves, even into modern times, like Marseilles in France and Syracuse in Sicily. But these colonies maintained contact with their mother cities and traded with each other and with the locals.

The best model for interstellar colonization was the settlement of the Pacific Basin by the East Asians, the conquest of half the earth's surface by a stone-age people in open boats. Check out "Interstellar Migration and the Human Experience" by Ben Finney for a thorough investigation of this idea. Hawaii was not discovered by Captain Cook; there was already a civilization there when he showed up. But keep in mind, Cook's Endeavour was the latest thing in 18th century European maritime technology. The natives got there on canoes built with stone tools from the meager vegetable resources of tiny oceanic islands.

"The Discovery of Hawaii", by Herb Kane.