The TGD collapse would irrevocably cement the idea that the CCP had lost the Mandate of Heaven.
Despite the Chinese in the People's Republic being officially atheist, there is still a lot of people there that still have Confucian and Taoist beliefs. The Mandate of Heaven is a big part of that traditionalist belief.
If the Three Gorges Dam collapses, we may have the biggest man-made disaster outside of war ever recorded. Tens, if not hundreds, of millions of lives are now at stake upstream and downstream of the dam.