You might also note that vast numbers of ethnic Tibetans live in areas which no one doubts are part of China proper.
Much of the discussion about Tibet is taken from the perspective of Westerners who assume that the nation state consisting of a relatively ethnically homogeneous population is, and ought to be, the politically moral norm.
The current boundries of what is taken to be "Tibet" consist of an imaginary line drawn by a British bureaucrat that bears little, if any, relation to the the historic realities of the Tibet/China relationship.