A lot of the recently printed books I have are printed on substantially higher quality paper than a lot of the older books I have - I really haven't seen that loss in quality.
A sewn cloth binding, archival-grade acid-free paper, and good inks will last centuries.
Pulp paperbacks may fall apart after a single reading, though I've got a few I've had for 20+ years and multiple readings, though a few of those have a fair bit of packaging tape on them.
Several of my college textbooks fell apart distressingly fast, and I've got a number of softcover books whose pages are getting unglued (varying vintages). Different cover materials can also have different longevities: a few composite / plastic covered books have had the covers crack off or break.
Exactly. They were made 25 years ago. I'm talking about books being printed right now. Quality is not what it once was.