answered:
what’s right about it? If it were such a beneficial, sustainable and lauded system of civic management, you’d think it’d result in stability and mutual flourishment rather than misery and insurrection.
If you’re gonna do “no ethnic groups allowed to mix” (not rate-limited mixing or frictional mixing or some-characteristics-but-not-others mixing, etc), then the way to do it is to give each group their own sovereign polity. Then they don’t have to fight you for control of “your” polity.
Of course, if one is planning to conquer stuff this isn’t really a suitable approach, but that conquest thing is not exactly a whole basket of ethics, itself.