[alkonaut]

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.

[bzbarsky]

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.