I doubt it would happen at runtime in iOS/Android, though? They would need to ship a JS interpreter that gets run on application startup, parse the React files,

I doubt it would happen at runtime in iOS/Android, though? They would need to ship a JS interpreter that gets run on application startup, parse the React files, instantiate the native controls and create a bridge between them back to the JS interpreter? Well on second thought I guess Appcelerator Titanium does something similar. So it's native UI only, but with the added overhead (JS interpreter + bridge, larger binary size, slower startup, etc.).