And also, the degradation that you see on sun-damaged clothing is mostly on the dye, not the underlying fabric itself.
Right, it's called lanolin.
It's also true that lanolin + shade and rain cover is better than lanolin alone. No protection is 100%.
> degradation that you see on sun-damaged clothing is mostly on the dye
"Mostly." We agree damage is done to the fiber. ;)
By design, modern dyes often act to protect fibers, soaking up energetic photons before they can damage the fibers themselves. In their absence, undyed fibers can be more susceptible to damage.
UV light will absolutely wreck most things, including fabric. Fabric left outdoors unprotected for a summer in Southwest US will be brittle and tear apart when handled. Plastic that isn't UV-protected will crack in about two summers. Hoses left in the sun will develop multiple leaks in 3-5 years, pretty much no matter how robust the marketing claimed they are.