Tibet was de facto independent for a while, but it re-joined China voluntarily circa 1950. About 10 years later, with some help from western agitation and assets, the theocratic ruling class felt too threatened by development empowering their serf population and sought to secede in order to maintain their fiefdom. Mao sent in the PLA and crushed the secessionist revolt.

You really can't "no u" this one because of Imperial China, the PRC's claim to Tibet is completely valid. You'd probably have more luck trying with Xinjiang, though evidently that is viewed as slightly played-out now.

In fact there are NO countries in the world immune to the “you displaced a previous civilization” argument. They’ve all done it.

Yet, some did it much more recently than others, and many of the peoples they stole from still exist and are still being exploited (not China-Tibet case). At the very least, one would expect countries not to go around questioning the legitimacy of other countries' territories when their own have no legitimacy at all.

Every aspect of human society exists because we, as a society, believe in them, including society itself. The very tools we use to measure the world beyond man are human fabrications. However, the maintenance of a social model does not necessarily depend on all its members believing in or agreeing with it, only on continuing to work for it. Cultural, social, and physical constraints exist and are very real. For example, it was common a few years ago in the "Free Palestine" online community to say that Israel is not real, but this statement has never stopped any of Israel's oppressive actions from happening. Understanding that all systems are fabricated is a fundamental step towards the possibility of replacing them with better systems, but for this to happen, realization needs to evolve into organized action: the only tool capable of changing the world. And yes, it is only possible to replace one system with another, and it is not possible to live without a system, because what makes us human is precisely this characteristic: we created the social system to overcome the evolutionary system.

And to rationalize the world we live in, we create rules to legitimize our other creations. We can use any factors to generate these rules, but to avoid chaos, we agree, in materialism, to use historical, cultural, and economic factors to justify the control of a territory by a nation state. Considering these factors, in a comparative sense, the control of the United States over any of its constituent territories is much more illegitimate than the control of Tibet by China. Does this mean that we should dismember the United States and return its territories to their original owners? No, it means that someone who believes that China should grant independence to Tibet should also advocate for the dismemberment of the United States. Since in this case the decision came from the United States government, which, I imagine, has no intention whatsoever of dismembering the United States, we can conclude that the only motivation for this is to antagonize China, and it does not stem from a concern for the right of peoples to self-determination.

Those dudes have a better PR department than the Catholic church

Catholic church isn't backed by the entire western propaganda machine, Dalai Lama is.

If that happens good? Why do we think having countries and borders that creates strife a good thing? If we eliminate all borders and are just humanity I see that as an absolute win.

If the country is a theocracy run by a ruling class of slave owning pedophiles, yes, that government deserves to go away. Serfdom is bad.

right to self determination

China freed the Tibetan people from serfdom imposed by a feudal religious state.