There are already organisms that feed on plastic, albeit slowly. See this fascinating 2013 article for an introduction: "Life in the “Plastisphere”: Microbial

There are already organisms that feed on plastic, albeit slowly. See this fascinating 2013 article for an introduction: "Life in the “Plastisphere”: Microbial Communities on Plastic Marine Debris" https://eportfolios.macaulay.cuny.edu/branco2014/files/2014/08/Zettler-2013-Plastisphere.pdf https://eportfolios.macaulay.cuny.edu/branco2014/files/2014/... Plastics also break down to simpler hydrocarbons and carbon dioxide with prolonged exposure to sunlight. Unfortunately, these gaseous small molecules are greenhouse gases, but they don't persist for millions of years. "Production of methane and ethylene from plastic in the environment" https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0200574 https://journals.plos.org/plosone/article?id=10.1371/journal...