My first thought is how accessible these books are. If a book hasn't been checked out in years, and there's another library in the interlibrary loan network that has a copy, there's no practical reason to keep another copy. If you can request a book and have it arrive in a few days, that's not an issue in any real sense, especially for books that nobody is checking out in the first place. I used to work in a library,…
I rarely go to a library to loan a specific work - I go there to find a work. This means going through dozens of potentially-relevant titles, taking them off the shelf, quickly browsing through them, and taking the one or two best ones home. This entire workflow becomes impossible if the book isn't readily available.
A book hidden in a box in the basement, or which arrives after only a few days, might as well not exist at all. I'm simply not going to scroll through a list, order several dozen books solely by their title alone, and come back a few days later (if this is even allowed at all): it's just not worth my time.
The whole "we keep a copy in a central archive" approach only works for historical purposes, not for actually making it available for reading. If you do that you have to also make digital scans trivially available for browsing - and in practice that rarely happens!