While that's a good point, I think that many libraries still have the overall design goal baked into them. These aren't independent libraries that solve an abs

While that's a good point, I think that many libraries still have the overall design goal baked into them. These aren't independent libraries that solve an abstract problem somewhere out in the aether, but rather libraries that are intended to fit into specific designs. Consider a serialization library, for reading and writing objects from memory. This library could aim to have a minimal memory footprint, and may fit into the design of an embedded device. This library could aim to have maximal throughput, and would best fit into the design of a server application. This library might aim for usability by new users, and handle its own memory allocation for the serialized object. But, that would imply serializing the entire object at once, rather than streaming it to disk or to a network socket, so that impacts what designs can use it. Libraries tend to be implemented such that they fit neatly into some original design. Figure out what that original design is, and that gives you a better sense for how it will fit into the design you're making.