"...if we cannot convict MP3 downloads of this "crime" against the music industry with the same burden of proof that we would expect of a trial in a court of law, then it is unethical and indeed unconstitutional to pursue lawsuits against individuals for a crime which cannot even be established to exist."
Of course, the industry doesn't need any evidence of declining sales from a legal standpoint- they have reason to believe that "making available" on a network is infringment. It's the hearts and minds thing they're after with these tears they shed over declining sales and their "decreasing ability to promote promising new artists", etc.
So you are actually battling them over a perception, not a legal point. I'd personally like it better if you'd left out the "criminal defense" angle and paid more attention to questions like just why exactly does Liebowitz stick his neck out drawing conclusions from such questionable data?