It was the right decision at the time - it made React much more familiar for people at FB who were writing PHP/Hack (Dan called this familiarity out in the post). It also enabled some great things that are still awkward with other patterns - https://github.com/reactjs/react-timer-mixin/issues/4 is an interesting case. That functionality is nice and elegant with mixins but not so with classes.

I think if we had to do it again (and get to keep the knowledge of the last few years), we'd probably skip mixins. We'd surely do some other things differently too though :)