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Amsel_ You could say they had an excuse to be Marxists still in the 60s, since communism hadn't shown its most gruesome side yet (or there wasn't full disclosure and recognition of the extent of human destruction it caused).
But if someone is still a communist after finding out about how badly that system failed in Eastern Europe (and elsewhere), then nothing would convince them except going through that failure themselves.
Some people will fight you tooth and claw for their chance to repeat your mistakes.
These people haven't even read their ideologues. Lenin, for example, believed communism in Europe was doomed if the German proletariat didn't start a revolution. And that's exactly what happened. Lenin was actually a very realistic ideologue, compared to today's communists.