As Porphyrogenitus suggests we might not phrase people as capable and not capable. Perhaps willing and unwilling might be somewhat better. If the USA were a largely freemarket libertarian society who would this country attract if not the willing what benefit would there be to the unwilling? In terms of open borders another countries willing would be expected to immigrate here. As some other country becomes devoid of its willing constituents it may be forced to change its policies as to attract the willing back. If the product is truly good for people at large I would expect the spread of freemarket libertarianism to equalize immigration to a large extent. If some such place becomes wildly lacking in population due to this I doubt some opportunity would not arise for use of the land, even as a commune for the unwilling.
Who should be truly not capable of supporting themselves is to further demonstrate the importance of the family dynamic and as also suggested by Porphyrogenitus the willing making eleemosynary contributions. I am personally not fond of giving to charities yet I am compelled to give from time to time.
When you consider those who wish to harm others, what is ever to stop them under any system? The best to hope for is that they would consider their freedom more important then their incarceration or death.
I think we have something pretty good with the right to life liberty and property but in order to be maintained must trump religious laws. Surely you could believe and practice whatever one chooses until the point of force on another impedes their rights.
I had not considered how we could change into such a society but Mr. johnsnow has helped me realize that this would need to start small perhaps starting with some state creating the example and how immigration and demand for living in said state could be the catalyst for change on a larger basis. I was always remiss to explain what could be done with all the expectations currently in place and how a change could be implemented outside of a complete societal collapse. So I thank you johnsnow and Porphyrogenitus for asking some important questions.
A thought on minarchism. I am not sure if a private or centralized legal/enforcement system is as key as how said system is enacted. If we could create the means to prevent the legal system from acting against a simple framework like life liberty and property then any such system should be fine.