Just like every other law? The legal definition of a monopoly has been refined over decades of case law, this ruling comes as no surprise to anyone familiar with the specific requirements of that definition (which were clearly not met by Epic here).
Well at this point there's only one higher court they could go to (if they'd even take up the case). Frankly the case law on this subject is pretty well established and the judges have just been following existing precedent.
Epic failed spectacularly. They couldn't prove a monopoly, and even if they could, they were not able to point to any instance where Apple abused a monopoly position to turn the screws on customers or developers.
M00x, you mentioned you know folks at Epic that are working on WebGPU. Do you mind sharing who they are? Because last time our team spoke with Epic, they mentioned they are not working on this (our company is a startup from Canada building tools and a platform-as-a-service to enable developers to bring their games to the web, and a large focus of our work has been building out a WebGPU backend and integration for Unreal Engine 5).
