It's certainly possible, but for now you could just write an app in plain React and wrap it in a web-to-native app wrapper, using something like Electron or MacGap (which Slack uses for their Mac app).
That would totally make sense. At Fb, we're focused on iOS and Android, but the community is absolutely welcome to step up and implement RN for the desktop platforms.
At least for Windows 8/10, I'm not sure if something like React Native is really necessary, since the platform itself already has built-in support for making "native" apps in HTML/JS.
There's no reason you couldn't implement it to work on desktop, but I doubt Facebook will spend the time. I hope the open source community implements this, though, because I'd love to have a truly native cross-platform framework that also works on desktop.