I don't think distributism requires a closed system.
A few of its guiding principles may answer your concerns:
* Subsidiarity teaches that things should be handled at the lowest or least centralized level that is competent to handle them. That doesn't mean that small organizations should be expected to bite off more than they could handle. It may very well be that a giant, multinational corporation is the lowest-sized organization that can handle operating something like an airline, and that is not inconsistent with distributist principles.
* Distributism, as an economic system, is not at all incompatible with things like a strong social safety net. However, it is worth pointing out that if the ownership of productive property were to become more widespread, the need for such a safety net would be decreased.
We live in a world where (to quote GKC [1]):
> Lancashire merchants whenever they like > Can water the beer of a man in Klondike > Or poison the meat of a man in Bombay; > And that is the meaning of Empire Day.
The advantage of centralization is that we can stop the poisoning all at once (because there is only one place doing the poisoning). However, when the meat is poisoned many more people suffer food poisoning than would if the food packaging were distributed.
[1]: http://www.gkc.org.uk/gkc/books/Songs_of_Education125.html
- People who can not work because they simply can't hold down a job, period.
- Externalities such as neighboring (or even distant) communes dumping wastewater into rivers etc., or polluting the ground, or air... you get the idea. What about natural calamities, such as earthquakes, tsunamis, volcanic eruptions, and such?
- Conflicts over scarce resources. Water, for example. Communes that rely on a common source of water will likely have conflicts. How are they resolved?
- Any commune is exceedingly unlikely to be good at producing all the things it needs. Trade will need to develop. Does 'self-reliance' exclude the possibility of trade?
- How large should a commune be, to enable innovation? Without innovation, there will be stagnation that will harm the commune in the long run.
- How would a commune handle people that are 'different'? How about criminals within and without?