And that's what it's all about: Energy Return on Investment. Conventional oil has a much much better return than fracking, and the US is out of it. This is what is meant by "decline". Sure, there's plenty of oil, but it's more expensive to get out, so we don't.

On the flip side, our economy is dependent on a certain EROI. Below that, it chokes because we don't have enough energy "slaves" to render the services we've came to depend upon.