What's your take on the article's take that they feel this plausible hypothesis is challenged by their study?
In particular, their study says: "The specific claim of the “linguistic niche hypothesis” that grammatical complexity should reduce with an increased number of nonnative speakers is not supported by our results." (from the source https://www.science.org/doi/10.1126/sciadv.adf7704).
If I'm reading their study correctly, they seem to suggest there was a somewhat arbitrary mix of complexity in root languages, and that modern languages just carry on that same complexity mix based on the languages that influenced them, which doesn't do a lot to explain why languages differ in "complexity" (except perhaps to argue that maybe it doesn't matter: any old language will do and people just speak like the people they learn from).