There's a very good point here - JS is being used for way more than just showing and hiding DOM elements anymore. It is becoming a full application language, and to that end requires the features that full application development requires, not just on frontend, but in Node as well.

I have just inherited a project written in 100% ES5 and I had forgotten how obscenely verbose JS <= ES5 really was. Every extra line of code is a liability and will slow down app development. Language features support precision and conciseness and that in turn promotes enterprise-level development. A toy language (old JS) can never support a full in-browser application.