They took a small, useful and elegant language, in part crippled by horrible, horrible, ugliness (e.g. for inner functions this points to the global object), and in preference to just fixing it, Javafied it.
This is not to say that ES4 should merely be ES3 spec polishing, but that ES4 describes a language not in the spirit of ES3. True, the spec ensures backwards compatibility, but in the sense of some new language that just happens to have ES3 support.
I think "javafied" is a bit unfair. ES4 continues the tradition from the original JS to copy Java-syntax where appropriate. "class X extends Y {}" looks indeed very Java-like, however the semantics are quite different, much more akin to classes in Python. But it would be silly to choose a gratuitously different syntax just to satisfy the Java-haters.
This is not to say that ES4 should merely be ES3 spec polishing, but that ES4 describes a language not in the spirit of ES3. True, the spec ensures backwards compatibility, but in the sense of some new language that just happens to have ES3 support.