As I inexpertly understand it, dissolving plastic and Styrofoam takes an extreme pH level, which then has to be filtered and heavily diluted before returning to the water table or bodies of water lest is obliterate the ecosystem. The whole process is apparently quite the ecological catastrophe, so it mostly doesn’t happen. Incineration, as you say, would flood the air with toxic chemicals, and is probably illegal in most places.
While half my tech and most vehicles would basically disintegrate, I’m wondering if the biohacker who releases the first plastovore microbe won’t be doing the planet a net favor, though the the law of unintended consequences could prove me very wrong.