The need for backwards compatibility is what is keeping JavaScript still not a great language. The number of wtfs and surprising behaviour is justified by needing to conform to what the language already did, and that is not a good thing.
I can't remember being bitten lately, maybe I have a blind spot.
These kind of things don't happen without such a person or group who will arbitrarily make decisions. The same reason why better languages like Dart don't get adopted.
There are too many client vendors and devices for this to work. And even with the new features in ES2015, almost everyone actually using them is still using Babel to support older browsers.