I for one do not see how imperfect people could possibly create a perfect society.
95%? really, you give credit to 5% for not having any greed? Natural Greed, the desire to have more, is a natural desire within humans, very few can resist it. We have a tendency to use the term only in relation to money, but it encompasses a lot more. For a communist society, or even a socialist one, to exist without conflict, greed would have to be wiped out. The fact that they rely upon such an ideal and the fact that greed is human nature, pretty much tells us that Socialism/Communism is not humanly possible and will always fail, or at least be constantly challenged, because they are trying to a society contrary to human nature. Greed is only one area of human nature that they fail to take into account, there are others.