Many people focus on the problem of getting there, to the RBE / Venus Project or whatever you call it, not questioning what to do when you finally get there. And all this under the excuse that current capitalist system is not sustainable in the long run (so we'll have to change or die). So let's see how sustainable Venus project is.
In a perfect world we'll have people with access to food and water, housing and clothing, top medical care for all. Now wars, no stress, many diseases eradicated. All this must lead to much longer average lifetime of a human. Using common sense, all of the people would be sure that all that will be available for their children for granted. They wouldn't have to work to support them, the system would do it. The children wouldn't have to work or worry about surviving without inheriting enough, since everything will be available to support their existence at least. Since we know that most people don't have too many kids mostly because of the problems of current society (greed for advancing in career, problems with supporting too many children), removal of those problems would lead to much higher birth rates per woman. And then with good medical care, infant mortality would drop significantly. So if we assume just 5 kids per woman (take an average for poor countries where people don't care about those problems with children in the modern society) we'll have doubling of world population every 15 years (rough estimate, don't judge it too much). Can the science and world resources really support all this, no matter how distributed they are (pay attention to the math - in 100 years we'd reach 1 trillion people)?
Or will we stop reproduction by some law? But then it contradicts the Zeitgeist spirit - to punish the offenders (how?), to force abortion or sterilization after 2 babies (talk about freedom?). One way or another perfect Zeitgeist world will break. Or you're so confident in the science taking us out of this planet and utilizing the resources of universe in such a short time?
You might not like capitalism (like I don't), but "invisible hand" does take care of this in a very natural way. As soon as we multiply too much for our technology to support it, we'll die in a war fighting for resources or out of hunger / thirst / cold etc. Nature won't shed a tear even if we all get killed in a nuclear war taking down all other species with us. It might be sad, but this is the natural way. Zeitgeist is describing an unnatural world and unless it plans on changing the nature I don't see how it will work.