While I don't disagree with your premise, "need to be an expert at both layers" is absolutely still true if you build your app natively for each platform (more so, even). At least with a common platform, if nothing else, you can share quite a bit of code, and that alone might make it worth the extra layers.
If you are already using React for the web (which I would believe is the biggest reason to use React Native), then the RN layer is pretty much already taken care of in term of expertise.
For the iOS and Android side, that's a constant between the two possibilities.
It's better because now your iOS and Android experts can concentrate much more on what they do best and your React experts can keep working on your UI like they always did.
I understood 'both layers' to mean both the abstraction layer, and the underlying layer being abstracted. Actually, you might even understand it as 'all 3 layers' here - iOS, Android, React Native.
For the happy path, you don't. And many apps can stick very comfortably on the happy path. I wrote a controller for a video mixer[0] that ran out-of-the-box on Android and iOS using websockets and everything Just Worked. (I still haven't released the iOS version due to lack of test equipment, but it runs just fine.)
