If a federal court holds that application programming interfaces (APIs) are copyrightable, the ruling will have no consequence.
An API is an interface, like the key slot in your car. It can be changed while still performing the same functions. An API can have things moved, changed and added without copyright issues because only the appearance has changed.
The reverse is also true: APIs, like key slots, can appear exactly the same, yet the under the hood, the mechanics are completely different. Copyrighting the way an interface looks and acts has nothing to do with the vastly more important issue of how it works.
Whatever the judge decides will not affect how APIs work and are used. Like a face-lift, things will look different while remaining unchanged under the makeup.