Sustained US sanctions on the USSR are what caused it to fall.
China did not become a market economy, its economy is still state owned, controlled, and planned. China, and Vietnam as well, have Most Favored Nation Trade Status. Without sanctions from the US as was put on the USSR, as well as being paid to manufacture everything for the US has made them quite rich, while the US slides. The issue isn't Communism versus Capitalism, it's simple math.
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People in Cuba and Venezuela do vote.