If I understand your hypothesis correctly, the obvious counterexample is German. Germany had a very weak central authority for most of its history, a steady influx of non-native speakers, and still maintained its complexity.
Mark Twain tried, but failed. As you probably know, he said he'd rather decline two drinks than one German adjective.
That was due to the bad luck of inventing the printing press early on, and congealing a grammatical mess before it had a chance to chill out and simplify.