I agree with your second point. But it would seem that normaly the languages that are spoken primarily by people that learnned them as second languages would become more simplified. And then tend to become complicated again as the speakers develop ideosincracies. But if there is a mechanism preventing this, for example its spoken over a wide area so the ideosyncracies never stick. Or the speakers are constantly interacting with forengers or both. Then the languaged gramar would remainsimple.

Persian gramar is much easier to learn than russian or spanish. And i asume chinise is likewise easier.