National borders are a hangover from an attempt to end the Imperial Tensions that led to the World Wars of the C20th. Britain and France were effectively forced to decolonise because Europe spent the first half of the century punching itself in the face. Living in the United Kingdom, there is very little self reflection about Colonialism having been a bad thing.
Woodrow Wilson pushed the idea of Self-Determination, believing "National aspirations must be respected; people may now be dominated and governed only by their own consent. Self determination is not a mere phrase; it is an imperative principle of action. . . .".
28th President of the United States
The borders of Empires were ever expanding, with the ethnic groups leading them often passing the baton. The Islamic expansions often conquered people only to have those people take charge. The Mongols shifted from Shamanism to Buddhism, Zoroastrianism, Orthodox Christianity and Islam, depending on where it spread.
The idea of a homogenous nation is bizarre. It grates me when I hear politicians saying the British People have voted for controls on immigration. That is rubbish. The Brexit Referendum didn't ask why people wanted change, or where they wanted to go. It just asked if Britain should leave the EU. People wanted to go left were on the same team as people who wanted to go right. I know people who voted for Brexit because they wanted free movement of people. They believed being in the EU meant that movement was restricted to Europe, and it should be open to the rest of the world too. I strongly disagree with how they chose to achieve that, but I too want what they want. I voted the opposite way. That is how confusing the question was.
The sentiment behind respecting aspirations and listening to people is a good one, but the idea that we are essentially different is not. Empires were bad in the way they trampled over people ignoring differences. Nations are bad in the way they fire up divisions and create boundaries between people that don't exist.
Apartheid grew on the back of ideas of Self-determination. Identity Politics. If the only way you determine yourself is through division and control, you are going to lead a lonely life. There is only one you. Even you don't agree with yourself. Yesterday you. Tomorrow you. Good mood you. Hungry you. Tired you. Excited you. All your yous want different things.