Well, you are right in that we seem to see an ever simplifying language - but there's also countertendencies time and again. Or as one of my linguistics professors used to say, "The Romans didn't step out of the cave with 6 grammatic cases."

There could be a trend, after dark ages of apocalpyse, be an emerging medieval culture that develops more advanced language skills, perhaps creating an elaborate manner of speaking as "modern" and to differentiate themselves from the "barbarians" who speak without rules. Though there'd probably be a high language conforming to all (new?) rules and customs and a low language of the street. Language has always also been a way to show your class or sophistications.