There’s a difference between a circulation library (your average community library) and an archive library (the Library of Congress).
Circulation libraries have the mission to serve the needs of the community, but have competing constraints. First off is space - it takes tax money to build libraries or enlarge them. Ergo, there’s only so much shelf space. Second, personnel. It takes librarians and library staff to operate, and the larger the collection, the more staff. That’s tax money that competes with the rest of the city budget. Need more firemen? Guess we won’t get that extra library assistant we desperately need to help cover programming.
Additionally, there is relevance of the material. Older works may be disposed of because they aren’t being checked out and newer books are in demand. This is not a travesty - that’s the role for archive libraries. Circulation libraries serve the active needs of the community, and if books aren’t being checked out, they aren’t actively serving anyone’s needs.
Multiple copies of new popular works are useful - when the demand for a new book is a waiting list 100 people long, you don’t want just one copy and they all have to wait for it. Again, the demands of the community mean multiple copies. And yes, eventually the demand drops and those extra copies are now superfluous space users and are, therefore, ripe for being weeded.
Scientific works are somewhat different than casual reading. They have value as the prime reference or as early observations in the field, etc. Again, it depends upon the library. A civic library has much less need to hold onto scientific works. A University Library has more need to preserve access to references, but even they face challenges of space, cost, etc.
As material becomes archived and accessible online, the driver to hold onto the physical texts in multiple libraries disappears. Anyone wants to look up that reference, they have access from their office or home and don’t need to traipse to the library to find the remaining copy in the back archives.
But, as mentioned, archive libraries serve the purpose of preserving works for the future.
Melbourne, I don’t know what you mean by “locked skips”.
Also, removing the book from the school library does not make the book inaccessible to Navajo wishing to find out what Americans in the 19th Century thought. But judgmental tones in books being read by schoolchildren can affect their self-esteem and pride in their heritage.