In a practical sense there are lots of things here that run in the face of this. Plastics aren’t necessarily a good source of energy, for example, so whether plastivore bacteria could ever practically decay plastics in the way you’re imagining self sufficiently is dubious. The main purpose of the wild modifications is to provide a means to digest, but that comes at the cost of energy in the enzymes being produced to do that. We see an overall economic benefit, but it may mutate out rapidly if it’s not actually providing a singular benefit to the organism.
Plastic generally already corrodes outdoors very readily. That’s the primary source of micro plastic is that exact corrosion. Those that don’t would be equivalently hard to digest.