It's true that ES6 classes don't cover all existing use cases yet, which is why I specifically mentioned in the blog post that we're continuing to support React

It's true that ES6 classes don't cover all existing use cases yet, which is why I specifically mentioned in the blog post that we're continuing to support React.createClass fully. The main Facebook site is still using React.createClass everywhere, though we are starting to use ES6 class components for some smaller projects here. As I mentioned in another comment on this thread, many mixins can be implemented as higher-order components that we call "containers". Many others relate to data fetching, so we're looking at adding first-class support for data subscriptions which can replace even more uses of mixins. (And in my experience, it's rare to add mixins to a component after it's written.) ES6 classes aren't suitable for everyone yet ("Ask your doctor if ES6 classes…") but they're a new API which is more convenient for some use cases, so we chose to add it now instead of waiting until we're at perfect feature parity to give people the chance to use the new syntax.