> But the rare books under discussion are closer to the end of their life and less likely to have been already digitized.

It is not at all clear that this is true.

The number of books published every year is growing rapidly. According to Bowker the number of books published in the US every year has increased ~15x in the past two decades [1].

Because of this, I suspect that the median age of the books we are discussing is below 30 years.

[1] https://www.writercosmos.com/blog/how-many-books-published-p...