Those things probably won't escape the atmosphere, however. It would be much harder to bootstrap an energy-hungry civilisation any time within the next few bill

Those things probably won't escape the atmosphere, however. It would be much harder to bootstrap an energy-hungry civilisation any time within the next few billion years, as we've used most of the easily-accessible sources of fossil fuels. If human civilisation falls, whatever comes afterwards probably wouldn't be able to build a rocket, unless they wait for fossil fuels to regenerate.