Earth's ecosystems have evolved biodiversity for exactly this reason: to recycle the waste products of life forms at all levels. They've gotten really good at this. There is still geologic evidence of this having gone way, way out of control, like vast limestone deposits, coal, and fossil fuels. Those were the unrecyclable byproducts of the day. Who knows what plastic will look like in the geologic record, but at this point they look destined to be a fine dust that will give future (non-human, likely) geologists a perfect marker of the Holocene.
Life optimizes for the most efficient resource usage. Those deposits were not an efficient source of energy.
But life itself is incredible, we have non-oxygen using lifeforms (inefficient, the earliest lifeforms), sulphur-eating lifeforms. Life can use up almost anything, <<if it has to>>.
