Other than just wanting to be able to copy other people's work as they often do all the time anyway, I'm not sure I understand major tech giants' argument as to why APIs would not be copyrightable. They are, if anything, much harder to make well than their implementations.
That is completely false. There have been, and will continue to be (even after this lawsuit) completely proprietary APIs protected by license agreements (often from even being published external to the agreement). If you can't copyright an API, many will go this route instead.