Sure it eats plastic. Until it gets a taste of human flesh.
It won't need to eat human flesh. What will happen is science will marvel at itself for solving the waste problem and it won't realize that the fungus doesn't distinguish between waste plastic and useful plastic. So when the landfills are gone the fungus will seek out a new means of survival: our homes and buildings. In a panic, the governments of the world will commission the creation an airborne pathogen to destroy the fungus. As it is released amid the crumbling infrastructure of society, science will once again revel in its own brilliance for saving humanity from yet another plague of its own creation.
But alas, science again doesn't consider all of the variables. Not only has their solution worked, it has worked all too well. Useful fungus begins to die, destroying the processes by which humanity processes food and creates medicine. Unable to feed ourselves, humans begin to turn on one another. Neighbor eats neighbor until all that's left of humanity are a handful of government officials and scientists. Unequipped to actually do anything useful, they wrap themselves in a cocoon of their own red tape and drift off into eternity. The planet didn't kill us. We did it to ourselves.
