The trouble with that notion is this: imagining that a plastic-based ecosystem arises (horrifying thought!) it means that there are life-forms capable of deriving energy from plastics, breaking them down. That makes plastics useless to us humans, because any time we try to use plastics for all the things we currently do with them, those life-forms are going to come along and attack, break down the stuff we deem "useful plastics"; the critters will make no distinction between nurdles lost on the beach and the plastics holding your car/house/clothes/aeroplane together. i.e. It's Game Over for plastics use.