Possibly for certain types of plastic, such as cellulose nitrate which is used for making celluloid film. I suspect it’s probably cheaper and easier to engineer the plastic than the plant.
The fungus eating plastic sounds like the basis for a hell of a movie. It gets loose and starts eating all the good plastic in the world, artificial limbs, cars, wow, it’s actually a bit scary.
Does it poop tiny plastic pellets? I know that biodegradable plastic generally does not deteriorate any more quickly than the others, instead breaks down into tiny pieces that you don’t see, but it is still there and will still pollute waterways.