Building colonies in-planet that could survive all possible scenarios would probably be impossible. But even if it were you'd face a pretty simple problem - who would ever want to live for there? You'd likely end up living in conditions that would make life on Mars look pleasant, without any of the upsides that might take people to Mars - adventure, ideology, commercial aspirations, perhaps even religious (you know the Mormons will want a planet or two), and of course 0.3g!
And who knows what the future holds in terms of population sizes? I also strongly disagree on the colonies not being able to engage in exchange. For a silly but very practical example sports in 0.3g are going to be insane. Jordan could jump something like 11ft and stay airborn for several seconds on Mars. That's going to be just be stupidly awesome to watch and play. MMA will look like a something out of a Chinese martial arts movie. For more mundane things, as the price of shipping cargo decreases the number of things available for trade increases. For example wine made in 0.3g will taste very different. Whether that's better or worse is yet to be discovered, but obviously such ideas will have no difficulty finding a market.
For better or for worse Mars (or the Moon) will also probably make amazing retirement places, especially if we can work on the scenery a bit. Taking that load of old bones might not only provide comfort but even increase longevity enabling a weaker heart to keep pumping a bit longer. And so on endlessly.
And who knows what the future holds in terms of population sizes? I also strongly disagree on the colonies not being able to engage in exchange. For a silly but very practical example sports in 0.3g are going to be insane. Jordan could jump something like 11ft and stay airborn for several seconds on Mars. That's going to be just be stupidly awesome to watch and play. MMA will look like a something out of a Chinese martial arts movie. For more mundane things, as the price of shipping cargo decreases the number of things available for trade increases. For example wine made in 0.3g will taste very different. Whether that's better or worse is yet to be discovered, but obviously such ideas will have no difficulty finding a market.
For better or for worse Mars (or the Moon) will also probably make amazing retirement places, especially if we can work on the scenery a bit. Taking that load of old bones might not only provide comfort but even increase longevity enabling a weaker heart to keep pumping a bit longer. And so on endlessly.