That's a bold statement. Why EcmaScript 5 and not JavaScript 1.4 for that matter?
Programming languages change and evolve. EcmaScript 6 introduces a lot of features, including some that greatly improve the readability, maintainability, and cleanliness of the code. Sure, right now you may need to hold back on some features if supporting legacy Internet Explorer versions is your goal and you don't use a preprocessor such as BabelJs, but at some point the only browsers that don't support ES6 will be browsers that are unsupported by their vendors and inherently unsafe; so why stick with ES5 in particular? Or are you simply waiting for the moment all current browsers support ES6?
I get not wanting to use bleeding edge features supported only in the latest developer edition of Chrome or Firefox, but at some point ES6 is supported in 100% of the browsers people can safely use, and a few years after that ES7, and so on.