The answer is mostly provided here https://stackoverflow.com/a/34210231/, but I'll recount it here for convenience:
Basically, you TC39 is changing the proposal so Babel is holding off on implementing decorators. If you want the Babel 5 version of decorators (or something similar), you can add this special plugin to Babel - https://www.npmjs.com/package/babel-plugin-transform-decorators-legacy.