I have many paperbacks from the 1950s to the present day. Some of the really old ones are a little yellow around the edges, but brittle and falling apart? Never. I have no idea what you are talking about.
How often have you read them? Many of my most beloved cheap paperbacks from the 80s and 90s have been replaced at least once since they've literally fallen apart as I was reading them. Also my books from the 50s and 60s seem to be of higher quality than books from the 80s and 90s (or that could also just be survivor bias).
My shelf is full of scifi/fantasy paperbacks from 80s to late 90s. A lot of these have their binding already broken. This happens especially to small (cheap) format books of more than 500 pages.
No problems with small books of reasonable lengths. At some point the number of pages in the popular books exceeded the durability of the cheap binding tech in use.
