That rule is simply an observation of the actions of notable* rulers in history.
How do we know that the person wasn't corrupt before they received power? Post hoc reasoning Meta? I expected more from you...
*(these corrupted rulers are notable because they received power and used it in a bad way, but those rulers that aren't notable are the ones who received power and didn't do much with it. I am wondering what would happen if we knew of all the rulers in history and compiled them into lists of: corrupted and uncorrupted. People in the middle would be judged by what their actions were for the most part, corrupted or uncorrupted.)
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While it is patently obvious that those who seek power ought to be the last to get it (I submit Exhibit A: politicians), I fail to see how "removing" those unsuitable for rulership would be an effective method.
For a start, the correlation of power to corruption is so high that the people who did well for their subjects are the exception (go on, start your list ... anywhere). Just look at the Holy Roman Emperors and Vatican Popes for a start, and they were men of God.
Secondly, removing all the people who would become corrupt once given power would novem-decimate (nonagintanovem-centum? nongentinonagintanovem-mille?) the population.
Thirdly, owing to the the dynamics of power politics, there is no way to assure that, given a new group where the most suseptible to corrupting power are removed, the new group will not spontaneously spawn new megalomaniacs, who see their chance now they are the most able to take control ...