You're assuming that the "cost in liberty alone is enough to render the whole enterprise a net loss." I'm assuming that copyright is a net gain to society. Since neither one of us can gather enough data to come to a rational conclusion, we can't prove which position is right. The best we can do is compare societies that enforce copyright to those that don't, and see what kind of results that are getting, assuming that all other factors are equal (and they're not).
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