Sure, that's something to consider before releasing stuff like this in the wild, especially if it can mutate and start attacking something else.
My point was: whether we use GMO or not, such a thing might happen naturally.
There is also a chance that such a mutation does not offer much competitive advantage if other nutrients are easy to find, and the mutation disappears. I wouldn't bet on it though, given the number of ecological niches that exist.
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.
My point was: whether we use GMO or not, such a thing might happen naturally.
There is also a chance that such a mutation does not offer much competitive advantage if other nutrients are easy to find, and the mutation disappears. I wouldn't bet on it though, given the number of ecological niches that exist.