Stasi was very efficient in suppression of German dissent.
East Germany collapsed on the account of persistent information inflow from the West (TV, radio, zero language barrier against West Germany), plus its ideological allies (the Warsaw Pact led by USSR) abandoning them. A country of 17 million isolated in a sea of non-friendly powers would be unviable. Once USSR dropped the Brezhnev Doctrine of Limited Sovereignty and Poland started introducing democracy, GDR was in a very tight spot.
China won't become isolated anytime soon, and there isn't really an efficient way to supply dissenting news to Chinese people in Chinese en masse. Unfortunately, I believe that Chinese secret service can stay efficient for a long, long time.