The original case, AFAICR had a number of amicus briefs on behalf of the argument that an API should not be regarded as copyright. That is because any piece of software using a library needs to incorporate at least some of the declarations contained in the API so that the constants, variables and methods used in the API can be referred to in the code.* If the API is copyright then nobody can write code against it without being granted a licence to do so. It would be a major impediment to software development. Software development custom has been to regard the API as being outside the scope of the copyright on the implementing code.
The original instruction to the jury by the judge - who appears to be not in the least confused - was to this effect and the jury so found. The problem arose because the appeal overturned this and, IIRC, SCOTUS refused to hear an appeal against that. This left a very serious situation for software developers - they would routinely have to infringe copyright simply to do their jobs.
The present decision is nothing more than damage limitation by making use of the API fair use. It's not ideal. If this is overturned then everyone's in trouble. The best that can then come out of it is that Oracle will find themselves in as much trouble as anyone else. If APIs are covered by copyright and use is subject to licence than that extends to the APIs on all GPLed code Oracle use. Oracle would then need to abide by the GPL and provide, under GPL, the source to any currently proprietory code which calls GPLed code. The penalty for not doing so would be revocation of the licence to use that GPLed code.
This is in danger of turning into an intractable mess if Oracle do win at appeal and SCOTUS don't overturn it. The best that can be said about it is that it would be restricted to the US (unless some other court accepts the US court decision as a precedent). It means software development there would become very difficult. It would also mean that it would become difficult to supply software developed outside the danger zone into the US; restricting the software market to the rest of the world. At some point the US would have to introduce legislation to overturn it. Ideally the Berne convention needs an addendum to clarify the situation.
There is, of course, the possibility that an appeal would review the entire case, overturn the previous appeal and reinstate the original jury's finding of fact. That would be by far the best outcome.
*Not universally true. Languages which allow identifiers to be used without prior declaration don't need it. Anyone who's made much use of such languages will almost certainly recall the bugs they created by simply mistyping a variable name.