I'm sure microbes will learn to eat plastic eventually. But if we don't mess with it, they have plenty to eat already, so the pressure to specialize on plastics is low, and we have plenty of time to live our happy lives before that becomes a real problem. But now we have tools to accelerate these processes greatly. And there's a real potential for us to screw ourselves up - and we are way not advanced enough to un-screw-up ourselves if that happens. We're like a person living in a paper city who discovered fire, but doesn't know yet how to make anything fire retardant. It's not a good place to try stuff just to see what'd happen.