One of the ways many web sites have grown is by cross-linking, cross-marketing. Little linking ads, rotated in many places, like those that link to Amazon from so many sites. Thousands and tens-of-thousands of folks become a sites "volunteer" marketing agents. Such a model could work with the swappers, if the swapping software allowed each swapper some provision for putting some marketing material in stream, in channel, along with the music.
File swapping is a golden opportunity -- it massively scales rapidly. The music biz's status quoism and greed blinds it to exploiting these golden opportunity without cutting off people's hands and hobbling techonology.
If exposure was the only thing of importance to record labels, you'd have a point. Online distribution would be golden. But the motivation for getting an artist maximum exposure is to drive sales.
If you've got wide exposure, but it doesn't increase sales, that exposure is nearly pointless. Should labels expend millions of dollars developing an artist's career simply to give away the product?
You've got to stop thinking so one dimensionally.