Yeah, all that plastic in landfills will likely disagree with and disprove your hypothesis. It is expected to be there for hundreds of years. This is why we push so hard to recycle plastic... It just doesn't break down easily.
Interesting example: One of the satellites I work on switched from aluminum supports for one of the antennas to plastic because the plastic holds up better in space. IN SPACE. Where it is constantly bombarded by radiation and charged particles from the sun and goes through an incredible temperature change (-80*C to +90*C) every 24 hours. The projected life of the satellite is 20 years but it will likely last 30+. IN SPACE. 22,000 miles away.