This is a meaningless argument. "Human nature" encompasses everything from Gandhi to Hitler. Nothing about human nature is universal or inevitable. Besides, Humans aren't the only species in the Federation. And as I pointed out, there has never been a true liberal democracy in history until the last 50-100 years. Liberal democracy is an experiment that we're undergoing now; history has no precedent for a truly egalitarian democracy. Which is not to say that it WILL last in real life, of course. Perhaps the human impulse for oppression and hierarchy will overcome the human impulse for liberty. I don't know. But in the world of Star Trek, which is after all based on the presumption that Humanity's political culture has fundamentally changed as a result of World War III and First Contact, and wherein Humans are only one of many species united in equal partnership under the Federation banner? I think the Federation may last indefinitely, in some form or another.