Everything is comparative. The major reason why China does not face more serious domestic unrest is that its international and economic progress have allowed it to appeal to Chinese nationalism. Withdrawing from Tibet would be viewed as an act of weakness, one which would do far more to undermine the Communist party’s legitimacy and support base than remaining there.  Secondly, attacks on China’s Human Rights record matter less and less each year as trade with the PRC becomes more and more valuable to the West. It barely affected the Olympics and increasingly it is viewed as an effort by the West to divide China.  Thirdly, the cost of the province has to be compared against the potential security risks an independent Tibet, especially one under anti-Chinese leadership, would pose to Chinese security.