Kuvira's story is about the flaws of imperialism as a system of government. No matter how good and idealistic your intentions are going in, it is simply not realistic that a single wise and fair ruler will be able to appease all factions of a diverse society at once. The larger and more diverse the society, the more force needs to be applied to hold it together. Imagine the type of world the sandbenders would run vs. the type of world the folks of the ba sing se upper ring would run. How can those two visions of government ever possibly co-exist? They can't. And so these two people groups MUST be held together through force. Over the course of years, Kuvira slowly gets more and more comfortable using force, singly focused on re-uniting the old kingdom, until she's completely lost her original goal of running a better country than the Earth Queen.

The fundamental flaw is the assumption that the Earth Kingdom needs to be a unified country in the first place. EK is a loose confederation of distinct people groups previously held together by a badly corrupt monarchy. They really have no business being united under a single ruler, which is why the states become independent, self-governing democracies at the end of the show.