But even a book from the 90s is 20-30 years old at this point. I'm not sure if that fits colloquially with a claim that they fall apart in "months to years". I've never had a paperback book I've purchased fall apart within 5 years (hell, I can't think of a time one fell apart within 10 years with the exception of severe water damage).
I just...have never seen a book fall apart in months.
I have. I had one fall apart on my first read through it. Pages just dropping out of it.
I also recall, as a teenager some years ago, buying a hardcover book from a store in the airport, and realizing that the last 20 pages were the previous 20 pages pasted in again.
Ultimately, the books I cared about re-reading, I've replaced with ebooks. Because aside from the convenience, I don't have to worry about the book's condition, deterioration, mold, et.al.
Although that was notoriously bound poorly.
I've had the binding snap on mass market paperbacks when I opened it the first time. Not exactly a shining beacon of quality.
The oldest hardback in my library is right around 300 years old and beyond a bit of foxing in quite good condition.
The 1898 Report of the Librarian of Congress has a long appendix title "The Durabilty of Paper" addressing just this concern:
<https://babel.hathitrust.org/cgi/pt?id=mdp.39015036735036&vi...>
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.
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 books from the 50s and 60s seem to be much higher quality than books from the 80s and 90s.
I'm not saying that your scenario can't happen. I am saying that I would be surprised if all but the most carefully maintained digital library would outlive the cheapest of print books. Keep in mind that, at a bare minimum, a digital library must be backed up and transferred to new media every few years. You may get away with storing digital for a decade untended, but two decades is a bit of a stretch. Also keep in mind that even the slightest amount of bit rot can make an entire book unreadable. While this isn't really true of books stored as plain text, most modern formats seem to used some sort of compressed container (e.g. ePubs are compressed).
I own tons from the 80s, all are in perfect condition. Also a few dozen from the 70s and they're all fine.
I also have a few books from the 1800s, those are hardcovers not paperbacks but also in reasonably good shape.
Keep in mind that bindings often last a lot less than that: One of the reasons genuinely old leatherbound books have those horizontal ridges on the spines is that they cover the (often also leather) laces used to re-bind the book and hold the pages together. This gave the classic works a certain look that was later replicated by publishers as just decorative ridges on the spine - but the origin of the feature was that those leather ridges were functional (they also acted as wear bumpers, but this was a secondary bonus) and a key part of the rebound book's structure!
I'll add one more big item to the anti-electronic book column: It is simply impossible to build electronics that last for decades with lead-free solder. Leaded PCBs will still eventually have tin whisker problems, but the new "green/RoHS" lead-free PCBs always stop working much sooner than leaded ones. Worse yet, no one cares because obsolescence design cycles are single-digit years now. Prior to lead-free electronic controls, appliances lasted many decades: Almost all of my major appliances are over 30 years old now, and some have never been repaired at all! They're a little less efficient, but much cheaper over the long haul, as I'm not replacing them every few years - that makes them arguably better for the environment, too, as there's no waste filling the dump, either...
Real books last centuries. E-books can't. We're a long way from Andromeda's "flexies"...