I won’t have to guess, beginning March 14 I’ll be teaching front-end engineering for The Iron Yard here in DC. If any of my students bring ES6 classes to me I will absolutely make them rewrite it using prototypes. This is for three reasons:

(1) If you can’t write it using prototypes then you have no business using the shorthand “class/extends” syntax. I feel the same about things like coffeescript and sass.

(2) While it is in the standard, it is not fully supported in the browsers engineers must support in most organizations. And yes, you can use a transpiler, but that introduces other potential issues (having to do with bugs in generated code).

(3) There is still the confusion for anyone ELSE reading the code. We don’t work in isolation, and just because the original author understands what “class” and “extends” means in JavaScript, the next developer to come along might not and might misinterpret what the code is doing and how to debug it or extend it.

So yes, if a developer (or student in my case) were to present code in a review using “class” and “extends” (and presuming I’m the lead) I would reject it and have them rewrite using prototypes.