That only works if all the libraries coordinate to determine which one will hold the last copy, and if the expense of moving such books around on request does not exceed that of storage.

Given the number of books I've been unable to find when I wanted them save in the Library of Congress (which won't loan, necessitating a trip to DC, or finding and purchasing my own copy), and the number of times my ILL requests have been turned down, a last copy per system mechanism seems the best for preserving access.

> That only works if all the libraries coordinate to determine which one will hold the last copy, and if the expense of moving such books around on request does not exceed that of storage.

Yale, Columbia, Harvard, Princeton, and NYPL coordinate on exactly this https://recap.princeton.edu/