When you say "It only works on paper", the usual implication is "It only works on paper, because it's failed every time that it's been attempted" even if you coach it in language like "human nature". But if the reason that communism has always failed in real life is that the US has tried to stop it from succeeding, that really says little about the hypothetical success of that system.
It'd be like if I said to you that bars "just don't work" in my hometown, because they always lasted a month before having to close due to lack of business, but disregarded the fact that I regularly firebombed bars in my hometown.
adding another myth to the mix:
"human nature is static"
or, if you prefer:
"human nature is independent from the way humans are socialized" ---