Does this help the JS engines? Instead of _knowing_ the developers intent and optimizing for it, it has to infer what the transpiler outputted and optimize for that. One seems harder than the other.
JSX by comparison seems much more stable than the class and decorator proposals, although arguably its features are much better optimized by a transpiler rather than a native JS engine.
There is a circular dependency: TC39 wants new language changes to be prototyped and proven in compiled-to-JS languages but the transpiler developers don't want to implement language changes that are not already on the standards track.