[retrac]

> I assume the argument here is that if you see a huge linguistic expansion and a huge demographic expansion occurring at the same time, you can reasonably conclude that they are the same phenomenon and the expanding language is spoken by the expanding people.

That's a lot to assume, though. One possible counterexample is contemporary North America. The English-speaking population spread across the continent in a linked process, the same phenomenon, but the majority contribution genetically, is not from England. There are communities in North America where almost no one is of English ancestry, yet they are English-speaking.

The same sort of social/demographic upheavals and changes that can create large language spread, are also the same kind that can create homogenization towards a single language despite people speaking many languages.