This presupposes that several thousand years from now 1. We've forgotten about fossil fuels 2. Fossil fuels are a desirable source of energy in the future 3.

This presupposes that several thousand years from now 1. We've forgotten about fossil fuels 2. Fossil fuels are a desirable source of energy in the future 3. Plastics buried in landfills will decompose into oil 4. There won't be more plentiful sources of oil, such as wells that weren't depleted in the present, or unrefined biomass that decomposed into more useable crude oil