Rumors of rock and roll’s demise have been greatly exaggerated for almost as long as there’s been rock and roll, so it’s not like this generation has a monopoly on rockist panic. But as today’s most popular rock acts cease to be rock acts — Mumford And Sons’ pop-folk, Eric Church’s country-fried wallop, the car-ad concept art that is Imagine Dragons — rock purists have found themselves increasingly rudderless against the Billboard chart’s current. While it’s true that rock has been losing ground with young people for decades, it’s typically been to other genres. Now the other genres eat it from within.