> Vanishingly few books are irreplaceable enough to need to be treated as special relics to carefully preserve for posterity

I wouldn't be so sure of that.

I always make an effort to buy books I really like (or expect to really like) in hardcover. This is because, around 20 years ago, I found some shelves full of books in a house belonging to some ancestors of mine. The books ranged from maybe 60 to 120 years old and had been unattended maybe for 40 or 50 years. At a first glance, all the books looked fine, but when picking them up, the paperback books literally crumbled in my hands and were unsalvageable. The hardcover ones were fine and I still have some with me now.

The thing is, lately hardcover runs of most books seem to be pitifully small. They print a few at the beginning and then they switch to paperback only. Often I want to buy a book that is only 5 or 10 years old, and almost no one has the hardcover. Those who do often demand extortionate prices. It's not uncommon to see regular hardcover editions of regular books with nothing special from 10 years ago, with retail price around €20, sell second hand for €100.

So if you consider that paperbacks aren't valid for preservation because they will crumble sooner than later, there will be many books with a rather small number of copies available.