Copyrighting api would be so unambiguously bad for tech as a whole that it would be better for society to repeal all copyright rather than accept this interpretation.
The entire industry rests on people making compatible parts. Pretty much all of software and most/all hardware currently used would become illegal in a fashion that would require negotiation with people many of whom are unknown or dead.
Want to reboot the entire tech industry while we shut down pretty much the entire developed world economy that can't operate without it?
We can also effect a global recession in which thousands in the poorer sectors of the world starve.
Or we can pretend this isn't real while the court fixes this.
Just because something takes work doesn't mean you get a government monopoly on it. Sewing patterns and recipies aren't granted copyright for instance.
Needs citation?
I don't know where to start, but no, this case ruling in Google's favor would not make code uncopyrightable. The general idea is that there is only one way to copy a method's signature. There are many ways to implement the actual logic. It's hard to find a direct source since no one else is jumping to this extreme conclusion. So the test is, could you have done it another way? And since you can not have a function with the signature System.out without having a function with the signature System.out, it would be considered fair game.
Why not?
Abstractly, interfaces are the ways you interact with a piece of software. Implementation is how the software works. That seems like a clear distinction.
You don't need to say "the interface is in the header files, the impl in the .cc or .cxx ones", specific facts of specific cases can be left to future courts. But it is absolutely possible to tell, in a generic sense, the difference between code and interface.