well, there are examples of anarchist and communist systems working every once in a while if you care to look. mostly, they function at the level of smaller communes which is what social anarchist though usually advocates.
this village has lived anarchist communist for a long time:
http://www.spectrezine.org/marinaled...r-world-exists
not saying that it's perfect but i think examples like these directly prove that it's not impossible. perfection will never be reached but we can always advance societally. the worst argument is that what we have now, a no longer really functioning form of capitalism, is the best possible system and the teleological end of the development of human society.