I don't anticipate a doom-and-gloom scenario if APIs were all-of-a-sudden copyrightable. Today, open source software is backed by powerful companies, and there is far less lock-in to any particular set of software functionality. The likely outcome is that big tech companies would just cross-license their basic APIs like Win32, SQL, NTFS, POSIX-like functionality, and other basic interface layers.
Today in 2020, the value is not in the interface, but data. Facebook probably wouldn't care if you created a clone that used the same API structure. But if you started imported their user data, you could expect a lawsuit.