The notion that there is something such as the purity of a language (English or any other) that can be preserved is entirely misconceived.
Let's go back roughly 1500 years to when Germanic tribes started to settle in Britain. They are believed to have come from different parts of what is now Denmark, Germany and Holland. It seems highly likely that the different groups spoke different varieties or dialects of the same language. This appears to be confirmed by the different dialects of Old English that can be seen from the texts that began to appear a few centuries later.
The next stage was the invasion of the Vikings. By the time they came the varieties of language that the invaders spoke had become differentiated from the varieties of language spoken by the Angles, Saxons, Frisians and Jutes who had settled a few centuries earlier. We can probably talk about the invaders and the "English" speaking different languages, but with a high degree of mutual intelligibilty. Many Scandinavian words entered the English language, most notably they, them and their and are.
The Norman Conquest did of course lead eventually to huge changes in English vocabulary. After that borrowings form Latin and Greek continued to enrich the language and we must not forget the influx of words from all over the world more or less starting with European colonialisation.
The position is now such that no one, however clever they are, can understand Old English without learning it as if it were a foreign language.
So, if you want to restore the "purity" of the English language how far back do you want to go? Even if you go as far back as the time when it could be said that "English" was a separate language from what was spoken on the continent, you still have the problem of which variety to choose.