Well, actually Classiс Rock's never gone away. Page and company are still alive, aren't they? And there are a lot of young groups playing old music and creating rock'n'roll by themselves. That's enough for Rock music to survive. But not to be widespread.
The problem is mainstream as it mentioned above. Who are the main consumers now? Teenagers. And I bet you they'll never choose R&R. They'll never feel it. Most of them. I'm telling that as a teenager living in teenage world. They choose only music they can dance to or music which makes them cute but never music which makes them think.