People forget, how oligarchies are actually not desirable for the oligarchs. Because there is no law and no stability. The zhar/king has a bad day and the whole crowd around you shifts in some economic landslide. Oligarchs in Russia came and went, and they took their money to europe/swiss/uk/us - because you can not thrust a oligarchy, when you are today in favor of the golden god king.

Such moves towards such systems, are usually desperate jumps of those whose empires are under threat of being broken up anyway.