I don’t buy that argument. For another, Anglo-Saxon isn’t a “pure” language. It’s heavily influenced by Old Norse. For another, for centuries after the Conquest English remained fairly impervious to borrowing, except from French. It isn’t until the 16th and 17th centuries that you start seeing borrowing from Latin or Greek in large numbers. And other languages pretty much follow the expansion of the empire. It’s the Anglo-American empire that has made English receptive to borrowing. Another factor might be the early-modern understanding that English was a minor, regional language spoken in a small, backwater nation occupying the extreme northwest of Europe, and that the nation had to look outside its borders for knowledge and culture.
I’m in the midst of collating all the OED data on origins, sorted by century, into a spreadsheet. I’ve just completed all the data entry, but it will be some days before I get to really looking at the data. For one thing, I’ve got to figure out a way to smooth out some of the more obvious biases in the OED citations (e.g., lots from c.1600 and the 19th century, damn few from the 18th or 20th centuries).