Is American mainstream rock dead?

No way!  As a matter of fact, here is Jon Caramanica's ringing endorsement:

"Declaring a genre dead is the worst, least imaginative sort of proclamation, so let’s call it zombified: it moves, it takes up space, it looks powerful from afar — with oodles of bands working hard, and some even making money — and garish up close. It lacks nutrients."

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The Year When Rock Just Spun Its Wheels