ES6+ is the way to go; with Babel transpiling and adding shims if backwards-compatibility if needed. These utility libraries will spread throughout the codebase

ES6+ is the way to go; with Babel transpiling and adding shims if backwards-compatibility if needed. These utility libraries will spread throughout the codebase and are very tricky to remove once they become obsolete. They often have subtile differences in behaviour to the ES6+ native constructs that further complicates removal. Babel on the other hand can eventually be removed somewhat easier, since you've been writing pure ES6+ anyway.