> those who don't use it are often forced to take it into account in their code; especially library authors.

No, `class` is syntactic sugar for the most common way of doing classes in ES3 and ES5. There is no semantic difference between:

    function Thing () {}

    Thing.prototype.greet = function () { alert("hello"); };

and

    class Thing {
      greet () {
        alert("hello");
      }
    }

They are the same, so it introduces no new traps for library authors.