Browsers aren't likely to implement two separate languages either. Furthermore, forcing people to pick between "ES5" and "ES6" if they just want to use a single ES6 feature is pretty bad.
So your fundamental choices are really an ES6 that is backwards-compatible or an ES6 that is not used.
On the other hand forcing newcomers to learn the warts of ES5 when they could learn what is becoming a reasonably elegant language isn't ideal either. Two scoping rules, for example.
An ES-latest runtime + transpilers would be all that browsers needed.
This has in fact been debated back and forth on the es-discuss mailing list and in TC39, especially given experiences with strict mode. The decision was generally made to not have more modes and to just have a single JS language.
You don't have to agree with that decision obviously, but this isn't something that happened willy-nilly.
Maybe even orders of magnitude more than that.