As much plastic as we dump into nature now, we increase the chance for a mutation of a microbe being able to digest it.

If that microbe manage to multiply and spread, plastic can start to rot, just like wood is doing now. On a long term basis, that might be a good thing, but in the short term it would be a catastrophy. We are using plastic as a long lasting material, often with no surface treatment. Imagine if wire insulation, waterproof encapsulation, gaskets and other important parts of our infrastructure starts to rapidly degrade!

That alone should be motivation for cleaning up and stop spreading plastic waste in nature.