That's a somewhat specious argument because there's no way to prove what you would or wouldn't have purchased. It's based on faulty premise.
Well, I can safely say that I wouldn't have even _heard_ of 95% of what I have if it wasn't for downloading random metal stuff from friends. And of that stuff, I have bought at least a dozen albums of my favouite ones, and I have gone to dozens of concerts and bought a bunch of merchandise from those bands.
If I hadn't ever heard of those bands (which I guarantee would be the case), then those bands likely would never have gotten my money.
The only time this thing doesn't hold is for the top 50 or so bands that are played on the radio/on bars/in movies/etc. They'll get a lot of sales because they have a big push behind them. But all the smaller bands who don't get radio time would be unheard of to me.