Why would it be random?
Perhaps I should have used the word "uncontrolled" instead of random. If you're expecting this bacteria to work against, say, a field with plastic litter in it, its going to be in contact with lots of other things made of plastic that aren't waste. If the bacteria is able to self replicate, and you've released it into the wild, you've effectively killed the use for nearly all plastics as things that are still in use will be decomposed.
We are already releasing huge amounts of artificial chemicals into the wild.
Thats a bizarre argument. That would be like saying: "We have lots of murders in the world, so why not intentionally murder more people?"