I've been thinking about this but based on the current rulings in the Oracle v. Google court case, it would seem that APIs are indeed copyrightable.

This would mean that Wine would be infringing on Microsoft's copyright of the Windows API and system calls. Of course it wouldn't matter until Microsoft actually does something about it. But as Wine gets better and better and its market share higher and higher, I worry that Microsoft might set their sights on Wine now that they have the law on their side.

As someone who is relatively removed from the programming world (I do basic Python scripting and not much else), I'm curious to see an argument opposing what I perceive as the majority viewpoint. Those against the acquisition have cited examples of Microsoft "ruining" services such as Skype and Minecraft.