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.
However, while some groups did flee south to other countries in Southeast Asia, most were eventually assimilated. Again, not necessarily disagreeing that those minority groups were initially considered secondary citizens, but obviously they eventually became part of the Han ethnicity--which is why there are more than a billion people who consider themselves Han Chinese.
And don't put words in hands (similar to words in mouth)--where in the comment(s) do you read that the "process has always blended ethnicities?" Oh, it's not there. You figured that one on your own--using your own biases?