Hopefully by now you are convinced of the utility of the enhancements ES6 brings. Unfortunately, it will be a while before you can rely on strong and consistent ES6 support on the Web.
So-called “evergreen” browsers are cutting into this problem; by automatically updating (without asking the user), they are allowing new web standards to be rolled out more quickly and consistently. However, this only reduces the problem instead of eliminating it.
Unless you can somehow control your user’s environment, ...