A country the size of China will not fall due to external threats, but internal ones. The Soviet Union was not militarily conquered but fell due to economic stagnation and internal discontent.
My friend, while those reasons did not help, those were not the root cause of demise of USSR (otherwise the west would have fallen long time ago).
By 1990, USSR's economic growth was not as fast as before, but there was no economic crisis like those in the western capitalist countries. Its human development index was among the first tier in the world according to U.N.
It was sold out by the officials who wanted enrich themselves by becoming capitalists. Vast state owned industries were privatized and those officials or their family and friends becomes the new billionaire class.
That's the real lesson that Chinese people needs to learn.