Bacteria acquiring genes to feed on plastics would lead to closing the circle again. Although supply will be cut short when humanity gives up producing plastics.
That resulted in coal deposits of the Carboniferous. Not oil.
It is only a matter of time before bacteria evolve some way of getting at the energy stored in plastics. Once that happens, once plastics start to "rot" as they are eaten by bacteria, the impacts on our technology could be catastrophic. The world might looks very steampunk as metal and ceramics replace plastics.
