Why can't 了 be used to indicate a completed action that has yet to come?
i think it is because of that we use 了 to express that the status is really exist,mostly it is the status of something has done or over/ended/finished.as you can not make sure a thing has not happened definitely happen in the future,so you can not say 明年，我学了中文.that sentence also indicate that you have finished the learning of chinese.