[Jedi Master Pikachu]

To be fair, the Chinese Imperial governments generally assimilated the peoples that they took over--by the method you mention, sending a lot of their people to live in the conquered areas, and then slowly taking over the people and area culturally. That's why Han Chinese are by far the largest ethnicity (which is not genetic, but cultural) in the world, while the ethnicity has a lot of genetic variation among them (i.e. both North and South Chinese can be Han Chinese).

[Wuli]

There was never anything "fair" about the "assimilation".

The "assimilated" group is throughout the period of "assimilation" rendered second class, not brought into commanding positions, not included in the best possibilities, economically or politically, until such time has passed that the fact that the Han had displaced (you call it "absorbed") the minority that the distinction was no longer obvious.

And you are wrong that this process has always blended ethnicities. Many of the current peoples of Vietnam, Laos, Thailand and Burma represent ethnic groups whose ancestors once resided in what is now southern China, until the constant attacks, militarily, and imperially after conquest, forced them to migrate south, in such large numbers that they in turn displaced the former dominant groups in those areas.

Just as today, the culture and identity of Tibet is being obliterated by what you call "assimilation". It is cultural genocide and nothing less. It has, and has never had anything to with the "security" of the Han. It's Han imperialism and it always has been.