But that's only for ILLs. CUNY's collections aren't that big to begin with ILLs are even fewer, especially in big cities with large public libraries like the NYPL, BKPL.

I worked at a much larger library system in the same city and I know for the fact that most do need to keep such transactions for years, at least for tracking and billing patrons for books lost or never returned. After another few years, they sell their overdues to (debt) collection agencies for a few pennies on the dollar.