I actually think this might be the only relevant thing here. While I personally greatly prefer writing JavaScript over CoffeeScript and would enjoy the consistency in an all-JavaScript code base, I don’t think there’s any significant barrier that would stop me or anyone else from contributing either way.
Given that the majority of the Discourse client code is already in CoffeeScript, if the core developers have a personal preference for it I don’t see a compelling reason to change course. It’s not as though you’ll be stuck if CoffeeScript suddenly “falls out of style” or something.
Granted, there may be something to be said about quality as it relates to JavaScript developers contributing CoffeeScript code that’s perhaps more JavaScript-like than it has to be, but I don’t think this is too large of a concern.