I would disagree with you in regards to the inevitability of an oligarchy. Rule and governance by a few with power (which is what oligarchy is) is I think inevitable in any society or organisation of significant scale, as per the iron law of oligarchy. As to ensuring an oligarchy is benevolent there is indeed no way to guarantee that. Thus the necessity of the voting system or if you happen to not be in a democracy revolution to reset the system. Oh and name for me one country which is not ruled by some form of oligarchy for example?