The introduction of "race" into the conversation is all your idea, which renders the rest of your response "rather pointless", doesn't it?
While it is true that during the millennia of their long relationship China meddled in Tibet's internal affairs - notably by, at times, deposing the Dalai Lama - until sixty years ago Tibet remained quite a separate entity from China. Chinese and Tibetans regarded each other as foreigners - and as far as I know, they still do. The stories that make it to the Western press are of good jobs going to Han immigrants, not Tibetans, and the Chinese I have talked to take pride in the fact that they have suppressed Tibet's native rulers, if not Tibetan culture itself.
I brought this up to warn you that you risk dangerously mis-characterizing the Chinese. I've met Chinese who have been quite humble and accomodating until they are appointed to a position of power over others; then they abused their position for personal and political gain. When confronted about this, one responded to me that his behavior was characteristic of his culture's value system and thus his conduct was not unethical.
So you see, China's rulers did not swallow Tibet until they possessed the means, through modern technology, to not just send armies in occasionally but maintain an occupying force permanently. They have proceeded with colonization since because there is no one to stop them. The humility you speak of is not itself a moral principle, but a mask that is currently useful, a tool to aid in China's interaction with us barbarians. When and where China's leaders want to expand (usually with the overwhelming support of the Chinese people) the mask is dropped, as in the seas south of China today.
Who knows if in a century or so China won't assert its trade links to Pakistan as justification to install its own favored leaders, and eventually to invade and colonize Pakistan itself? After all, the Himalayas aren't much of a bother to airborne divisions based less than a thousand kilometers away from Pakistan's capital.
