The sad reality is that it adds zero value at trade or resale. Not saying it’s right, but there is no value assigned to it for buyers.
I kinda disagree... It may not add that much, but it adds a bit. If I am buying a used one, and I see two identical cars one with full PPF from a top brand that looks neat, shinny with flawless paint, and another one with normal swirlk marks or chips for it's age, I would be glad to pay at least 1000 USD more for the one with full PPF.
Now, depending on where you live the labor cost, you do NOT need to always pay 8K for a full PFF. In some areas, it is no more than 4K (smaller towns and states wherein labor hourly late is not as high) for a top PPF brand. PPF film does not cost shops that much, it is mostly labor, and can be done in one day for a good pre-cut templates... Hence, 4-4.5K is completely reasonable to expect. Of course if you live in FL or the bay area, good luck getting anything for less than 7K.
So, in summary, if you can get full PPF for a good price using a top brand film, why not!!!... I would at least gladly pay 1000 more for a neat flawless car with full PPF if not more, and I have done that in the past. Now not everyone would do that, but in the private sale market, you for sure will get just some money above fair market value for a full PPF or full wrap.
Also, keep in mind that many of us are NOT doing PPF for the protection, but simply to have a car that looks sooooo glossy and shiny with no swirl marks whatsoever. I hand wash my cars, and with PPF, I am guaranteed to have a car with paint that looks even better and more glossy that factory even after years. Llumar Valor has 12 years warranty and covers any seam that lifts or any rock chips that goes through the film. Majority of rock chips that do not fully penetrate the film can self-heat (some won't) but in most cases you are guaranteed to have a better looking vehicle, and most importantly, easy to clean. That's why I got it... it is for me, not the next owner. I can't tell you how much I love my PPF each time I look at it under the sun when clean and shiny. It really adds a lot of gloss and character especially on blue paint. Maybe not so much on white, but darker colors or blue, it adds a lot of extra gloss. Not "necessary*" but it does add a lot of benefits cosmetically for sure. Worth it? that is subjective there is no right/wrong answer.