Until, say, they mutate independently. Now, I have no idea how likely that is, but I think the fear is not baseless. There's a fun SF book, "Directive 51", which explores this a bit. Not a bad book to cuddle up with.
In a natural environment this will not hold. Organisms are unstable; they change all the time, simpler ones even more so. Mutants with a faster growth rate will, up to a certain maximum, take over from slower growing or growth-deficient mutants. There isn't really a compelling growth advantage for plastic-degrading organisms, which is one reason why plastic stays so long in the environment.
All pre-processing steps further add to the cost, so we will year after year hear about it - and then wonder why nothing happens and plastics keep compounding in the ocean. And this is not new either: https://en.wikipedia.org/wiki/Diamond_v._Chakrabarty
Jurassic narratives notwithstanding, unsure if this is a real problem. If you have a yeast that will only survive under narrow unnatural circumstances, I would expect freeing it into the wild with natural circumstances means it dies - not uses nature to find a way to destroy the world as a form of poetic justice for ever having been created.
on edit: my confusion stems from being the first to reference anything Jurassic in the conversation, so you sound somewhat accusatory.