I was the director of radio for broadcast.com, which was actually "AudioNet" for the first few years I was there (we changed our name when we went public).
Streaming for OTA station was to help build TSL, just give you more opportunities for your audience to listen to your station, its percentage of listening is still very small, but slowly growing, but it won't be a 'threat' to OTA radio listening on a radio.
For internet only stations to work, they need to be able to have individual ads which address each individuals likes/wants. A ad that is aimed at the general public, won't have a high enough
CPM to make it work, however if you can serve ads for a boat dealership for people into boating, and ads for "plastic widgets" for people that like plastic widgets.
They have said for many years that was possible (it involves registering users, getting more information, etc),,only then will Internet station really be able to make money, look at Pandora, they have to limit how long a person can listen if they aren't paying for the service, the advertising revenue wont' support it (the more they stream the more money they lose.
Internet only stations have a place, especially among younger audiences,,,but there is a lot of work to be done before the business model works,,,someone like Apple and Google getting into that business is a little scary, because they can spend the money necessary to overcome those hurdles,,but it still will never replace OTA radio.