I think this is a fallacious expectation. For most environments, I don't see why there would be selective pressure towards plastic degradation. That is, "burning" plastic as fuel would likely require the absence of an alternative. Obviously this hasn't happened with coal or oil, although those are energy dense fuels. There are already organisms which have the enzymatic means for the break down of some plastics. Given the omnipresence of plastic pollution, if there was selective pressure, we would probably have seen some specialization already. Especially for microbes. It took a looong time before fungi were able to breakdown lignin. Before that, the only thing removing dead trees was fire. So... there is a good chance we are going to see shit all. Especially on land.