The main threat to this illegal but widely practiced activity is legal action by copyright holders against individual traders. The only effective protection against these threats is the barrier that could be provided by anonymity. An effective, anonymous file sharing network would see rapid adoption and would be the number one driver for widespread use of anonymity.
If I thought I was being ripped off by anonymous file sharing, I'd try to push legislation that would mandate registering beforehand any download volume exceeding x per month. Downloaded more than x per month but not registered? Then you'll have to lay open your traffic, including encryption keys.
The reasoning would be that most people won't have any legitimate business downloading more than x per month. By adjusting x, you can make a strong case. Once you get this enacted, you first get the ones with huge download volumes; then you lower x and repeat until the number of false positives gets too embarassing.
If that seems drastic, just take a look at other legislation that has been enacted recently. I certainly believe that it's possible.
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