Basically: The product of a human being's work is rightfully his property; that's part and parcel of living in a civilized society. Doesn't matter if that work is entirely "intellectual" or not. The fact that it's physically easy to steal intellectual property does not make it right.
People who make arguments about natural rights typically also think that if people's natural rights are not respected, that is going to ultimately be a net loss for society. I agree with that in the case of intellectual property. If people's intellectual property rights are not respected, it will not be profitable to develop intellectual property.
Although we shouldn't shun abstractions, it's also important not to shun concretes. In this case: you claim that it's important for the public good to move works into the public domain. Well, how is it good for the public to move Horton Hears a Who or Sports Illustrated into the public domain? Much less Waiting for Godot.