Chinas problem will be all of the old people that are alive but not contributing and it will be sudden. Their economy is all about making things and pushing it onto other countries. When they can’t do that, either because they don’t have the people, the supply chains are broken, or other counties can’t afford to buy their widgets, their situation looks bleak and immediate.

That's exactly what I was talking about with the demographic transition models. Both South Korea and Japan have demographic transition models where the age imbalance is already worse, and both are worse today than China is predicted to be by 2050. This also isn't just about gross exports, the biggest factor in the decline is going to be infrastructure, medicine, and other critical workers. When you have a population that is mostly comprised of elderly who are unable to contribute to society, you also need to account for the detriment that poses to society at large. That places necessity in medical care (which is already a huge problem in the modern day) and other necessary jobs, such as building and maintaining infrastructure, operating markets, etc. The GDP is only one of many problems with population decline.

Unfortunately the USA does not refine its own oil. It sells it and imports and refines foreign oil. Changing or replacing oil refineries stateside to refine domestic crude would take billions of dollars and years to construct.