You're mistaken about the nature of Stalin's takeover of Eastern Europe. He maintained plausible deniability until the very last minute. He made it look like a democratic process while his cronies were throwing opponents out of windows. Some people did see through it, people like Churchill and Patton. But Roosevelt and Truman were completely fooled by Stalin, and thought they could trust him. https://en.wikipedia.org/wiki/Eastern_Bloc#Concealed_transfo...

As for communism proving non-viable, how can you possibly judge actions in the 50s by what surprised everyone in the 80s? And we have every reason to believe that communism would have spread further and lasted longer if not for the extraordinary Western efforts to prevent and undermine them. They would certainly have taken all of Asia and South America, for instance.