Actually, you make a really valid point, but it's leading me to an unfortunate conclusion.

If bacteria was developed that could break down plastics and did actually break free to start digesting plastic in the wild, that would really clean up the earth, for maybe 10 years. But then the manufacturers would start using "more durable" materials, they'd slightly change their plastic chemistry to resist these microbes and we'll be in the same boat we're in now, where plastics don't break down.

Because the thing is, one of our primary uses for plastic is when we want a material that won't rot or rust, so breaking down defeats is purpose. We want plastic to not break down, so our products continue to work longer. But we also want plastic to break down, to not pollute our environment. These two goals are completely at odds with each other. The result is, and I now 100% believe this, we will never really have bacteria that breaks down plastic, long lasting plastic waste is a product of our own desire for "quality" products.