If the advertisers don't agree ... that's all that matters. A station can have huge online listenership. If they can't sell it to the national advertisers, the listeners don't matter.
The other thing is that there are other ways to monetize a stream besides :30 agency spots. A few companies have gone in that direction. The future is very clear. It's going to take some time, and it will be messy, but the future isn't in towers and transmitters.