Plastic-eating whatever evolves in environments of extremely high concentration of plastic and unavailability of anything else. As soon as anything else is available, the plastic-eating thing evolves back to eating something else.
If you had nothing to eat you'd also give plastic a try, and you could miraculously be the chosen one who can break down plastic, but you'd switch back to normal food as soon as possible.
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.
If you had nothing to eat you'd also give plastic a try, and you could miraculously be the chosen one who can break down plastic, but you'd switch back to normal food as soon as possible.