I think plastic-eating bacteria being released in the wild would be a benefit if it was isolated to only target plastic... even if we lost the ability to use plastic as a material due to this stuff floating around I think it'd be a net benefit... but whenever anyone starts talking about specialized bacteria I get the willies - that's messing with dangerous stuff that could result in an existential threat.
Nah, if plastic-eating bacteria were released to the wild, it'll be something between a huge annoyance and a civilization-ending disaster. Plastic is a wonder material, and while we definitely don't give it its due respect, it plays crucial role in places like hygiene and energy efficiency. Imagine if you suddenly had to worry about all the plastic things around you rusting.
Also yes, engineered bacteria and viruses are dangerous stuff, but that's the closest we have to a self-replicating robots today. I don't know what else the OP could have meant, besides making a joke. That said, if we change "self-replicating" to "cheap and mass-produced from recycled material", then the whole thing starts to make more sense.