"It took a lot of fossil fuels to forge our industrial world. Now they're almost gone." - it's wrong from the start. We have hundreds of years of fossil fuels available with current technology. With future technology, it could be even longer.
This sort of misses the point that under this thought experiment there is no future technology, and that to get at the vast reserves we have left would require that technology. We won't be deep sea drilling or fracking if we get rebooted. Nor will we be digging holes in the prairies and having crude gush to the surface.
So... in a post-apocalyptic scenario in which civilization has regressed to unindustrialized agriculture, there would be fossil fuels available only with 500+ years of Industrial Revolution advancement?
I'm not sure how that would be helpful.
Factor the cost of combustion into the cost of extraction, and you'll find that our useable reserves are rapidly approaching zero. Indeed, we may be in overshoot already.
http://www.nytimes.com/2015/04/09/upshot/if-we-dig-out-all-o...