You have stated "For some reason, however, we don't seem to apply this sort of necessary-but-not-sufficient thinking to systems above a certain level of complexity... such as ourselves". The theologian spends a great deal of time with this question. Witness the life of Martin Luther and others who have considered which of righteousness, the works of righteousness, grace, and faith as being necessary but not sufficient for salvation. To an unbeliever I realize this exercise may be one of futility, but to the Religious it is an all consuming task and the answer(s) are considered to have eschatological consequences.