I disagree with your first two sentences, human leadership was irreplaceable until very recently, perhaps only 10 years ago (I THINK longer, but I'm not sure the networks were robust and widely-distributed enough until the late '00's), and sociologically nearly the entire planet is incapable of recognizing how miserably corrupt human leadership inevitably becomes, regardless of the quality its inception.
A flat democracy would be practical today, but not actually rational. Your merit-weighted democracy is quite rational, but would probably trigger at least national, if not global, violence to try to implement, as every US citizen has been brainwashed from childhood into the idea that their innate rights (and I don't gainsay they are innate rights) translate directly into intelligence and competence to govern their own lives properly, and by extension, the rest of the world should follow in lockstep.