There are no straight lines to the past. There is no definitive translation of past institutions to the present. The present, as separate from modernity, demands change. That is not to say the leftist drift is inevitable (though it is likely) but to say that economics, technology, culture and environment change an institution and the way it can and will operate. The advantage of monarchy it is a simple system that even the most daft of humanity can grasp. That is if they are not inculcated with innate hate by 12 years of propaganda sessions. It is easy to set up all one really needs is power, money, and/or military, and a family.
Now one must eventually create a Schelling point around primogenitive succession, but that is not the subject of this post. The problem with translating Monarchy to the modern age is the untold wealth available. The principal advantage of primogenitive monarchy is that the King has an incentive to keep the country going for his own wealth as well as the wealth of his children. When the majority of the country were a couple bad harvests away from starvation there was a strong incentive to not screw up for the king. His wealth was limited and his capital could be spent very quickly. This did not stop kings from making bad choices or inflating currencies or pissing off the peasants, but there was a reasonable assumption that at the very least within a generation or two the feedback from bad decisions would force some sort of change.
The problem today is that most countries have the potential to generate more capital wealth than any one person could possibly spend or want to spend. In the modern age being the king of some back water country is reasonably cushy, it is not really that much different from being the king of a rich country. Certainly if tomorrow there were a king of America they would have way more wealth than they would know what to do with. Billions is more than most people could possibly imagine spending. A modern king whether he was good or bad at running a country could reasonably assume that he would be wealthy for the rest of his life and probably at least three or four generations down the line. Clearly the incentives due to the differences in the capital society can generate produce very different incentives.
So how do we solves this problem? I know you have plenty of ideas so let’s make it harder. How do we solves this problem without fundamentally changing the nature of monarchy? What can we get away with and still call this system a monarchy? One easy solution is make the state smaller, less wealth better feed back mechanisms. Monarchy was designed to handle less wealth and less people so why not reduce the scale. A second solution is give the king a fixed percentage of the wealth large enough to make him happy but small enough to threaten his next vacation if 10% of the economy suddenly went down the drain. A third solution is to play on the King’s ego. If you associate national wealth (social capital, culture and other non economic achievement should be just the majority of the measure of “wealth”) with the King’s virtue, intelligence and greatness in a religious manner (note this could be secular but whats the fun in that?) and the King believes it he has an incentive to keep society great. This could easily back fire if it defaults into central planning. One must balance the orthodoxy of the King’s greatness with incentives for him not to stray to far into central planning. One way to do this is to give the king the power of law but not of the purse. Make him spend his own money of extra programs, outside of the essentials (defense, law, order). Though relying on a government even one person to follow the rules is a bad idea. Generally speaking any modern system should stay far, far, far, far away from any sort of centralized banking system, though having an accepted list of currencies that the government will accept isn’t too too bad. This would create artificial subsidies while allowing commerce to flow in whatever currency was fashionable. The structure of the modern world may not be kind to such an ancient institution (monarchy), but none the less it would probably out perform the whims of the demos. There might be a few crazies, a couple steps back, but no one person could reproduce the backwards insanity and inefficiencies of a modern democracy.