Napster, at least (I don't know about Pirate Bay), also enabled a class of underground/amateur music by giving it a distribution channel that never existed before. In a way that iTunes does not. I think, at least to some degree, you have the causality backwards. People consumed mainstream content because that was what was largely available to them. How much of mainstream content consumption has been replaced by amateur content on the web? As the culture at large gets more and more used to this, I think the idea of mainstream will hold less and less power. Of course, unless devices like tablets continue to convince them that the mainstream is the only valuable stream.

The content is available on tablets, too, but the device subtly privileges its own centrally mediated distribution channels. I believe this privilege will only get stronger.