Monarchy is rule by a single individual. It works on this wise. Immediately after his succession, the new monarch enthusiastically attempts to rule the country. For a certain period, shall we call it a year. As there is only so much time between breakfast and supper, this is largely impossible. The next year, he carries on out of a sense of duty. The third year, he announces that he does not want to be bothered with this ruling crap, but if there are any fit women around would you please send them up. Monarchy then gives way to pornocracy: porne is Greek for prostitute.
Previous objections to Monarchic rule which I have rejected rest on the possibility that the monarch might be an idiot or a psychopath. In my estimation, mere idiocy or psychopathy are less damaging to good government than politics is.
The commenter brings up the more fearsome possibility that the monarch could be a normal sane bloke, more concerned about what his girlfriend thinks of him than about whether GDP next year will be 2.5 trillion or 3 trillion.
If decisions simply end up being made by some random attractive woman (or boy) instead of the hereditary ruler, that's not a problem in itself. But the reason this situation is so much more dangerous than mere insanity is that it produces politics, (meaning a struggle for power), based this time not around arming supporters or controlling journalists, but around forming close personal relationships with the monarch. This was often the main form of "politics" in historical monarchies.
I'm not sure that it is a worse form of politics than exists in a democracy or a military Junta, but my aim in proposing monarchy is to remove politics altogether, which is obviously more difficult than I thought.