Well, for one, the argument was based on Amazon sales numbers that have since been discredited. For another, even if it applied to books, that doesn't mean that it would apply to everything. It's quite possible that a few things (like books) have been limited largely by real world constraints and were waiting for someone like Amazon to come by and free them from the 80/20 rule, but that most things were not.

People bandy about "the long tail" as if it's some sort of conclusive proof that conventional economics is in the process of being shaken up by these new distribution methods, when in reality that's just conjecture and doesn't yet seem to be the case.