It's a common mantra amongst environmentalists that "Plastics are forever" - meaning that they never degrade or otherwise disappear.
There have been a recent flurry of reports to suggest otherwise. The Chemical Heritage Foundation has a 3-part series about how plastics are not forever, with the focus on artworks made from plastic. In some cases, the artwork only lasts a few decades. (The articles are obviously not written by someone with a chemical background, as they made some chemically inaccurate statements about acetate leaching out of cellulose acetate, but the overall gist of the articles is still accurate.)
In the same vein, Sarah Everts of the Artful Science blog at C & EN News recently asked the question: "How long should conservators protect David Beckham’s football?", with the focus being on how long special footballs and other artifacts should be kept. Will people still even know or care about Beckham in 100 or 200 years? Considering the ongoing costs for properly protecting artifacts, at some point, the decision needs to be made to let them go. While the authors mentioned that the proteins in the leather skin will eventually degrade, the rubber bladder allowing for inflation of the ball is equally at risk.
Plastics are not forever. Plastics are not diamonds . They aren't some magical material that never degrades. They are just another organic material subject to the weaknesses of all organic materials. Unlike diamonds, plastics burn. They can be easily torn. They yellow upon extended exposure to sunlight . They crumble after exposure to ozone and have a whole plethora of other weaknesses. Diamonds are forever; plastics are not.
 Technically speaking, even diamonds aren't forever as thermodynamics demonstrates that they are in a slightly more energetic state than graphite. However, the energy barrier is so high that the kinetics of the transition are negligible.
 ...or darken or pinken or...depending on the exact formulation. (Yes, I did say pinken.)