More than a decade ago, I had started my career, then everyone was busy writing database-centric, server-side applications like web services, web applications, websites, and many more using Java, .NET, PHP, ruby and other technology stacks.
Also, I was told that JavaScript is meant for web page manipulation. You should manipulate web pages only. At most, you should send AJAX requests using JavaScript. It was a language of the browser ONLY.
Of course, co-founder of The Stackoverflow had something else in his mind.

After a decade, everything has been changed. The application that is created using JavaScript is targetting to the server, desktop, laptops, tablets, mobiles and almost everywhere. It is now language beyond the browser.

We could write web applications and services using JavaScript using server-side environment like Node+ExpressJS+mongoos.
We could write single page apps using various JavaScript frameworks and libraries like Angular, React, Vue, Ember, Backbone and many more.
We could write desktop applications using JavaScript like Slack, VS Code (The IDE).
Some of the tools that we use to develop, transpile, bundle, minify, serve and many other operations on JavaScript apps are also written in JavaScript.

In short,

However, some of the developers that are new to JavaScript don’t like some of its features or syntax and choose to go with ES6 to develop their apps. They think ES is the new cool player in the market over JavaScript.

These people try to use ES features without knowing JavaScript good parts. They don’t know ES6 is a makeover to JavaScript.

You should know what is right and wrong about ES as well as JavaScript.
Eventually, some of the makeover features of ES6 transpiles to classical JavaScripts. For that, you require to know about JavaScript fundamentals.
Please see the following examples from React. One using class and another using pure function.

Same outcome can be achieved using pure function that transpile to less complex code.

You would end up like these people who don’t know how to use tools at hand.

So, choosing right feature at right time is important.
The fundamentals of JavaScripts would help you to achieve it.

Hence not only next-generation ES6 (or next) but also JavaScript fundamentals, knowing Good and Bad parts of it, is also important.
Resources:
- JavaScript: The Good Parts – Book by Douglas Crockford (Fundamentals)
- You Don’t Know Js (6 Book Series) by Kyle Simpson (Grow as a JS dev)
- https://www.ecma-international.org/publications/standards/Ecma-262.htm
- https://developer.mozilla.org/en-US/docs/Web/JavaScript