I doubt the other languages are "dying off". Javascript isn't the most elegant language but it's certainly getting better with ES6.

I've been building a React Native app and there are a couple big positives for it.

l. All the web developers at my company can manage in JS. You don't need special talent or training like you do for the other languages. Our native apps are built by completely different teams because of their native skill sets.

2. The app we're building shares most of its code between iOS and Android. This prevents you from building the same app twice in two different languages (and as I pointed out, for us, in two different teams).