I'm not really sure what kind of angle you are trying to take with this. I really don't think you can attribute in full, or even a decent part, of a country's economical outcome over a period of time to its form of governement, especially if you are only accounting for its current standing. History has provided monarchical governments plenty of wealth and "superpower" status throughout history. I'm not sure what you mean by "why does it need a monarch or the rejection of one". A governmental change is most likely the cause of an economic change, not the specific government entirely. Rejection of a monarchy doesn't guarantee economic growth and neither does the injection of a monarchy in a depressed economy.
Maybe you are looking at this in the wrong way and creating conclusions based of your perception? You might find more enlightenment by studying individual governmental structures and their histories rather than trying to create a puzzle by fitting together pieces from various puzzles. Monarchies can be very efficient forms of government since the power is centralized, the problem lies in who runs it and the fact that power is switched over every so often because people die. Even the greatest ruler in the world could have a selfish cruel offspring. This applying only to inherited forms of monarchy. So monarchies tend to fluctuate. Republics (like Rome and the modern "Democratic" countries) tend to have a more stable development cycle which can provide a greater benefit and thus allow a country to last much longer than a monarchy, but even they aren't invincible and can still be taken down if the right circumstances occur. Even Rome understood the benefits of the monarchy structure and thus provided the ability for a Caesar to have full power, though they might have failed to place proper safe guards in.
I'm not sure if economy is the greatest factor in determining the benefits of a governmental structure. I think you are applying much too strong of a weight to that, especially if you don't consider countries that aren't "superpowers" or in the top tier of wealth, successful countries. I'd say "superpowers" are more like the freaks of economical strength, and they don't stay that way. It's kind of similar to how some people think that games that sold less than Call of Duty are failures, but its obvious that's not the case.
Before the PS3 everyone was nice to me :(
