The Chinese Communists seized power in 1949, and invaded Tibet the following year on the argument that "what was once ours is ours forever." So long as they hold power in Beijing, they will also hold Tibet — but an interesting analogy comes to mind. For the history of the Baltic states — Lithuania, Latvia and Estonia — is not very different.
They fell under the rule of the expanding Russian empire in the 18th century, but regained their independence after revolution overthrew the tsarist regime in 1917. They lost it again when the Soviet Union invaded them in 1940 — but got it back when the communist regime in Moscow collapsed in 1991. And the main motive for their drive for independence was fear that their languages and cultures were being submerged by a wave of Russian immigrants.
As with the Baltic states, so too with Tibet. If there is ever a change of regime in Beijing, then a window of opportunity will open — and Tibet will have a couple of years to establish its independence before a new government emerges in Beijing that feels compelled to hold onto it in deference to Chinese nationalist sentiment. But that window is not open now.