It is important to note that the Tiananmen Square Massacre happened just before the fall of the Berlin Wall and the collapse of communism in Eastern Europe and the Soviet Union. The Chinese communists are well aware of the unintended momentum their crackdown gave to other peoples in the world.
There is a large number of Chinese forces already amassing on the border. If they enter the territory to impose order Tiananmen-Square-style, they won’t be using rubber bullets and bean bag rounds. The protesters are perfectly right in their demands, and they are right in their fury at the brutality of Hong Kong’s Police Force, but they must now exercise that virtue which is stereotypically Chinese: PRAGMATISM.
Mainland China has had an authoritarian government for thousands of years, and demanding recognition of democracy from them at this stage is about as much use as asking a tiger to eat lettuce. Against China, they cannot win, and if China enters Hong Kong to impose order, the cost will be unimaginable.