If you already know ReactJS, then it'll probably be a lot easier for you to learn React Native than Swift or Objective-C.
If you are working on a team with primarily JS developers, React Native is probably a better choice to go with.
Will this cause Swift to be irrelevant? Probably not.
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If you are learning iOS and doing production / client work, you should be aware that using a third party framework could actually break your app. I'm interested in React Naive, but also very weary about it. Facebook released a framework called Three20 back in iOS 2.0/3.0 days and abandoned the project shortly afterwards. Of course being open source there is bound to be people who take over the project, but it's something to think about.
