How say the philologists here?

It's pretty much in line with what linguists and anthropologists have been saying for years. Remember, language is not genetically determined. Folks pick up another language quickly if it's valuable to do so. The Latin-speaking Romans did not replace all the Gaulish-speaking people in what today is called France, but the latter did learn Vulgar Latin quickly. The reverse is true when the Franks invaded. Their Germanic language left an impact on the language, but by and large the people kept speaking their Romance language(s).

Ceci n'est pas un seing.