It's the same dynamic as with antibiotics resistant bacteria. Given enough time, something will probably appear that does not pay a high price to digest plastic

It's the same dynamic as with antibiotics resistant bacteria. Given enough time, something will probably appear that does not pay a high price to digest plastic (or can even use the mechanism for other material), and doesn't evolve back when other food is available. If it's bacteria, then soon the entire world will be full of plastic-digesting bacteria. If it's fungus, it will start to appear here and there at random.