Plastic gets consumed by oceanic microorganisms, though slowly. Take a look at the fascinating microscopic images of plastic partially eaten by microorganisms in this 2013 research article, "Life in the Plastisphere: Microbial Communities on Plastic Marine Debris":
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 as long.
"Production of methane and ethylene from plastic in the environment"
People tend to care disproportionately about human life. We don’t actually know whether we’ll adapt at the pace required to overcome widespread plastic contamination (or really whether we even need to adapt).
I think the idea we have this highly dense energy source pervasively available across the earth in all biomes with an exposed surface area that is unfathomable, life will adapt to consume that source of carbon rich energy. I suspect we may be bringing the end of the age of plastics rapidly.
And the lifecycle of the likely consumers of this energy source have much faster reproduction cycles, therefore faster ability to evolve to use this energy source.
I'm very curious how we will protect against this, otherwise bacteria will eat away at our car dashboards and such.