This map says it all:
The Tibetan plateau is key to controlling the headwaters of just about every major river in East and South Asia. China controls Tibet, it controls most of the water that it and its neighbours depend upon.
As written on this blog for years, it was always going to come down to this. It's all about water, who can take what and the nations that will emerge was "water haves" and "water have-nots." Right now it's China that holds the whip hand. And Tibet is the key to China's water security. End of story.