On the subject of Tibetan assimilation: what most people think of Tibet - that is the current Tibetan autonomous region of the PRC - is actually only about 1/3 of what many Tibetans would call "Tibet."
What is culturally and ethnically Tibetan actually stretches across Qinghai province, as well as large swaths of Sichuan, and part of Yunnan and Gansu. If you go to these regions and talk to Tibetans, they will tell you you are in Amdo, Kham, etc, even though you may be in the administrative province of, for example, Sichuan. Residents will still sometimes even use these designations on their postal envelopes.
There are some historical reasons for the way Tibet is currently defined, but "Tibet" was also chopped up into a much smaller contemporary "tibet" for, er, strategic reasons.
If you are interested in the big 3 Tibetan regions:
https://en.wikipedia.org/wiki/Amdo