Some of the plastics today are very tough and there has become a real science in developing new and stronger plastics. But at the end of the day they never outlast good old-fashioned steel and cast materials. Plastics inevitably become brittle with age and although some steps have been made in recent times to UV stabilize some plastics, the solution is by no means permanent. Add heat, petrol, oil, grease, sunlight, vibration, and abrasion to the equation, and the potential life span is even less.
Incidentally, I am amazed at some of the PVC "welds" available in tubes these days. I welded up a broken piece on an engine fin with some of this newfangled "goo" and after it dried sanded it off. It is still going strong after a couple of years. I mixed it up on a piece of metal sheet first before applying, and the leftover went so hard it took several hard blows from a cold chisel and hammer to knock it off. Even then it did not come off cleanly. In fact it appeared to be almost stronger than the steel underneath, or had at least "bonded" with it. Tough stuff indeed! One drawback with it is you need to start with a very clean surface. Any oil or dirt greatly compromises its weld ("glue") ability.
