That's a straw-man argument. The question isn't whether file-sharers are net positive or negative. The question is whether if file-sharing were reduced, would the industry be better or worse off. It's entirely possible that if file-sharing was more risky or less accessible, the people who file-share would spend a lot more on music.

I'm pretty sure that the RIAA has never stated that file-sharers are bad for the music industry, just that file-sharing is bad. Those are two different things.