even once all the language features you want are in, Babel is amazing for other things such as optimizations, transforms, injecting assertions depending on environment, inlining stuff, static analysis, etc. If you use Flow its used to strip out annotations, and if you use JSX (not just in React) its great there too.

We get a lot of millage out of it even when our target platforms support all the language features we need. Your millage may vary.