I remember coming to the same realization when the Google Vs Oracle court case was going on. Which decided that API’s are not copyrightable. To me it felt like the wrong decision, but what do i know.
I would definitely argue that public interfaces should not be copyrightable, since you need to be able to freely reimplement them to interoperate, which in my opinion should be preferred over granting even temporary monopolies to software interfaces.
However, public interfaces are only a small part of software interfaces in total. It's getting the system's internal interfaces right that takes most of the work.
