Anything that is on display is going to fade more quickly because of light than something that is kept closed.
When I was a kid in the 1980s I collected many mass market paperbacks (expected to be ephemeral) from as far back in the 1960s, even in the early 2000s I thought these held up pretty well, but circa 2020 I think many of them are getting pretty bad. (Contrast that to trade paperbacks that are sometimes "acid-free" but that frequently break in the first minutes of use because of incorrect and inconsistent construction.)
My house is humid and not a great place to store books, but I went looking in an academic library that follows "good" practices and found that mass market paperbacks from the 1980s and earlier were in bad shape too.
When I was a kid in the 1980s I collected many mass market paperbacks (expected to be ephemeral) from as far back in the 1960s, even in the early 2000s I thought these held up pretty well, but circa 2020 I think many of them are getting pretty bad. (Contrast that to trade paperbacks that are sometimes "acid-free" but that frequently break in the first minutes of use because of incorrect and inconsistent construction.)
My house is humid and not a great place to store books, but I went looking in an academic library that follows "good" practices and found that mass market paperbacks from the 1980s and earlier were in bad shape too.