The plastics chosen for lenses are incredibly resistant to UV light due to many additives and often powder coatings, and in general incredibly dimensionally stable. This is actually when even very expensive lenses often use plastics inside, because those plastics don't expand/contract/deform as much because of heat as metals, leading to more precise optical alignments and higher image quality.

Ironically, since these lenses often need much smaller adjustments and don't need as much to be readjusted after taking them apart, I find it easier to repair them than some complex metal lenses.