So there's a lot mixed in here.
1.) Plastic can become brittle due to temperature/UV radiation, but that doesn't mean it always is. Also, the "worn" surface doesn't occur because of embrittlement, but the material is exposed to wind (particles), water, chemicals, etc. etc. And it just wears out. It's the same with metal and wood. Glass and paint are a little less sensitive, but if you look at the windows and paint your 20 year old cars, you will probably find that they are no longer as good as new.
2.) Plastic, glass, and metal have different coefficients of thermal expansion, but it doesn't depend on the color of the window. Dark foil MAY increase the surface temperature, but since plastic is a good insulator, this only occurs in layers close to the surface. Also, plastic gets hotter on the outside than it does on the inside of the house. In addition, those who object to expansion here should pay attention to the difference in the expansion coefficients of glass, plastic and aluminum. If a difference of a few degrees through dark film was a problem, then no window should be tight, and glass should fall out all over the place.
3.) Of course, some plastic windows no longer look like new. But, as mentioned, they have only been standard for 40 years. Metals and wood have been used for centuries. In plastics, development has advanced considerably since the 1970s, while other areas of materials have developed little. In other words: plastic windows from the 80s, even if they are still white, are no match for today's standards.