[davidktr]

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.

[mcswell]

Mark Twain tried, but failed. As you probably know, he said he'd rather decline two drinks than one German adjective.

[euroderf]

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.