You think government tyranny comes from a lack of trust of citizens? I think that's just the excuse government gives.
Discussion
When the people of a society are culturally and biologically 90+% similar, as Americans were in my youth, their values and moral behavior are also very similar, so they trust each other not to bring bombs into airliners or concerts, or put poison into children's Halloween candy or juice bottles at the grocery store -- and that trust is amply justified. In such a society, tyranny isn't at all necessary to preserve order and peace, because order and peace are largely spontaneous.
Government will bring tyranny regardless of the race uniformity of the people. Just look at China. The lack of trust due to a mixed people is just an excuse, but government wants authoritarian power regardless.
True, they can bring tyranny and often do, even in countries lucky enough to have escaped multiracialism. But it's much, much harder to unite and launch a revolution against tyrants if your fellow "citizens" are of 100 disparate backgrounds. And it's much, much easier to (mis)rule and oppress a squabbling congerie of tribes than a united singular people.