Actually coffee's js is currently compatible down to IE6, which the author said is what he wants to support at this point. So unless MS were planning to backport ES6 compatible JavaScript to the dead IE6, CoffeeScript supporting ES6 will either change/break at some point or become a unreasonable backward-compatibility clusterfuck (probably the former). ES6 becomes official in late 2014 so there is probably enough time for a lot of changes to happen.
Are you saying the current JS loops will stop working at that point? Of course not. So why would it matter? CoffeeScript -> Javascript will continue to work like it always did.
current js loops for sure will not, but I guess maintaining full backward compatibility would be a hard task, so I assume at some point CS will evolve to support only the newer standards. ie6 compatibility it's something jeremy talks about in the podcast I linked in the other comment here.
