> Does a library have no duty of preservation or archival? Surely "what the public wants" cannot be the only driving factor, right?

Yes, it is the only driving factor for a public library.

It turns out people do want to preserve stuff, so we keep an archive of local interest items that the public wants (microfilm, city directories, notable local authors and artists), but we don't view ourselves as an archive. That's not our mission nor are we equipped to do that work beyond the limited capacity we already have.

The same goes for materials in the collection. If they do not check out, we remove them and replace them with things that hopefully will. Years of experience has taught me that people do not want an old collection, no matter what its value. They want current items that are clean and relevant to them and that's what we're here for. We're a government institution and are ultimately answerable to what the public wants.

But here's the thing, like with our local archive, the desires of the public and preservation do often align. That's why libraries look the way they do. People do check out great works of literary art and we often replace those as they wear out. People do want some archival materials and ask for them when doing research. The situation you describe doesn't happen because it turns out most people don't want all gardening books or adult magazines. But we don't keep anything that doesn't get used just for the sake of it, all of it serves the mission.

>Years of experience has taught me that people do not want an old collection, no matter what its value. They want current items that are clean and relevant to them and that's what we're here for.

With all due respect: if this is true, how come it feels like I can never find any recent-ish literature in any given library? be it a rural area or university, I feel like trying to find anything more recent than 20 years old or so just doesn't happen. And technical books evolve quickly.

maybe it's a domain problem.

I think this is a problem specific to your library. A very quick check shows the average age of our collection to be about 6 years (at my small to midsized urban library). That's extremely rough because new editions of classics will show a much more recent copyright date than an average person would consider them, so a collection can still be seen to skew much older than the data suggests. However, it's still dominated by recent titles, especially of popular literature. Technical books are harder for me to comment on because at a public library, that's generally out of our domain. We try to keep up on the popular series for tech stuff like "Learn Visually" or "Dummies", but the circulation or those dwindles as either fewer people need them or are more likely to use the internet for solving their problems. Depending on your needs, you might see if your library has a subscription to O'Reilly books or similar platform for your area of interest.

Public libraries have no such duty, no. Outside of the largest systems, most aren't fit to do this in a way that would materially achieve the goal of preserving anything, either in terms of space, equipment, or expertise.

So generally speaking, providing "what the public wants" is the entire purpose of public libraries. Obviously librarians exercise a lot of discretion about what specifically to buy and make available in the library, but most have long since moved on from paternalistic notions of needing to provide access only to The Great Books.

The library of congress has a duty of archival, but public libraries are there as a service to the public. So, what the public wants from them is actually the only driving factor.