In the end, nothing is made of standards like 25 + years ago. If liners lasted 20 + years, companies would not be in business. I can just tell you that my old front door needed to be painted because of how quickly the sun wold fade the paint. Eventually, the door had to be replaced. Now, every 3 months, no matter what I add Armor All, and the color is still there 6 years later. When the shine starts disappearing, I add another coat. I think if the industry could figure out a way to protect the liner, one would be an overnight millionaire. There have been articles that claim that higher chlorine levels fade the liner, but I would assume that the dye and pigmentation would be removed, more than fading.
The sun is very powerful and destroys things very quickly. I left a Home Depot Orange bucket outside for the winter. The bucket was destroyed from the sun in less than 6 months. I still feel that the sun is the biggest culprit.