You never actually need 90% of the functionality a library gives, though... With some notable exceptions (cryptography). Most times I've excised a library I've only needed to implement a small part of what it did.
That's true, and there may be valid reasons to remove a library and reimplement that 10% yourself such as for performance, stability or educational reasons. However if the library is performing as expected, does not using 90% of it make it any less valuable? If the problem has been solved in a satisfactory way and remaking it doesn't bring benefit or solve a problem then it seems wasteful to spend the human time to do so