And, yet, that bacterium isn't consuming and breaking down all the PET around it (It was isolated outside a bottling facility, after all. It should spread like wildfire.).
Why?
Because the energy expended to break down the plastic means that it is outcompeted by the bacteria who get their materials by other means unless plastic is the only thing available.
In the ocean, there is always lots of other things available.
Perhaps this will turn out like the evolution of lignin-modifying enzymes, but those took a LONG time to come about even while lignin is far less stable than most plastics.
> That's just not very likely to happen from a basic chemistry point of view.
Actually, it has already happened (DOI 10.1126/science.aad6359, http://science.sciencemag.org/content/351/6278/1196). However, do note that there are many types of plastic (https://en.wikipedia.org/wiki/Plastic#Types) - the linked article is about degradation of PET specifically.