"Bergstrom took issue with Yamaki’s suggestion that paper is less biodegradable than some plastic bags. “Plastic does not biodegrade; their chemical makeup does not allow it,” he said"
From what I have read
i) paper bags are harder on the environment than plastic bags when you take into account the manufacturing process.
ii) the plastic bags that we use to have do completely grade, in about 20 years.  I don't know about the thicker ones.